I do want to die but I don't want loved ones to be blaming themselves or anything.
Questions:
How can I minimise the effect it will have on them? (I thought about doing it on 29 feb because it's once every 4 years)
The way I think of this is basically as an exercise in estate planning, the only difference being that most people who do estate planning don't have to make decisions about when or how they die. So, I found an estate lawyer, got the will and other documents put together, and got general estate planning advice from him. I think that's a good starting point. It's a lot more work than I thought it would be, but what I've done so far:
Pick a good date, away from birthdays, anniversaries, holidays, etc. that loved ones care about. Feb 29 is pretty genius, but for my money, my birthday would be ideal, since that's going to be a painful anniversary after the fact, no matter what (and my birthday is far enough away from all the other dates loved ones care about). Might be a good idea to have a few dates or date ranges picked out, too, in case the ideal date doesn't work out for whatever reason.
Same goes for location, pick one that isn't meaningful, that loved ones won't have to pass by or be exposed to, etc. CTBing at home is problematic for this reason. Give thought to who will discover the body - best that it's not a loved one... as far as I've worked out, best option is a first responder (police, fire), who get paid for and are used to having contact with dead bodies.
Consider that a loved one will almost certainly be the one to identify the body after death, so physically traumatic methods (GSW, jumping) or methods leaving a disfigured body (drowning, not being found for more than a few days) will result in a painful identification process for the loved one doing it.
Do as much of the estate work as possible before hand so that no one else has to. Get a will, organize, pack up/throw out all your stuff so family members don't need to spend a lot of time sorting through what's left behind physically after the fact. (My estate lawyer advised selling everything, which is worth considering, depending on how certain one is about being imminently dead.) Same applies to online life - write down account and device access information and get it to loved ones. A good place for that is with the will/estate packet. If possible, make sure all your assets (retirement savings, life insurance, investment accounts) have the proper beneficiary set, rather than relying on the will to sort that out. (Again, cashing those out to a bank account is worth considering depending on one's level of certainty.) Also consider that things in solely your name will require a death certificate to be transfered to your beneficiary, so I was advised by my estate lawyer to set up joint accounts where possible, which lets the other party access the funds without a death certificate or waiting for probate on the will. (If it comes up, the proper answer to an estate lawyer is along the lines of being terminally ill but not wanting to talk about details. A lie about a plausible terminal medical condition, just in case might be a good idea, too.) My estate packet, in addition to a will, includes financial power of attorney documents, a DNR and living will - having loved ones agonize over whether or not to have a braindead body hooked up to life support seems like the worst thing possible. Where I live, it's not legally binding, but advisable to include wishes on matters of the funeral and disposition of the body as well. My estate lawyer also suggested that I could prepay for funeral and burial expenses, which is a good idea if you want those things. I personally don't care at all, so I made a point to say so - funerals and the like are for the benefit of the living, not the dead, so anything or nothing at all is fine by me, whatever they want.
Pay off bills and credit cards where you can, and cancel recurring bills and services, if possible, so the estate isn't charged after death and loved ones don't need to worry about that after the fact. I'm on a prepaid cell plan, have disabled 2 factor authentication on my accounts, and set security verification options to the email account my loved ones have login information for.
Give thought to how to notify loved ones and which people those might be. I've got a texting app on my phone with the capability to schedule texts, for example. That allows texts to go out after death, but before one of those horrible death notification visits.
No matter what, suicide hits loved ones extremely hard, so there are going to be "mental health" issues after the fact. In my area, there are a couple good resources for suicide survivors (people who've had a loved one CTB), and I made sure to provide contact information for those suicide survivor support groups.
How do I write my suicide note?
There are a few resources here on writing a good note, as far as content goes. The big things I've pulled out of what I've read are to apologize (because it will hurt the loved ones), to absolve them of blame, to assure them nothing they could have said or done would have changed things and to avoid anything that might be taken as a "final wishes". For example, I read an account once about someone who was a talented piano player going to some prestigious music university. One of her parents CTBed, and in the suicide note expressed a hope that she'd be able to continue on with her dream of being a successful professional piano player... and when she couldn't make that happen, had years of crippling guilt over not being able to honor her dead parent's final wish. Sounds like something to avoid at all costs.
How to best do that entirely depends on individual circumstances, obviously. For me, I emphasize the pain and misery I've been in for decades, acknowledge how incredibly loving and supportive the people in my life have been, tell them how much they've meant to me and apologize for it not being enough, and have personalized notes for the close relationships I have, instead of just one note for everyone.
As far as the note itself goes, in all likelihood, the cops will seize the note and not release it until the death investigation is complete, if ever. There are plenty of heart breaking stories you can find online about grieving families spending months trying to get a copy of a suicide note from the cops.
Accordingly, it seems unwise to rely on the cops doing the decent thing (imagine that), so the traditional approach of just having a suicide note on the body is not a good one, as far as I'm concerned. I see four other options - postal mail, a delayed email, including note(s) with the will/estate packet and saying what you need to say ahead of time. All these have benefits and drawbacks, and it's ultimately a very personal decision as far as which way or ways to go.
How do I make sure my belongings are given to my family?
With a will. As mentioned, my estate lawyer advises all his terminally ill clients to sell everything they're not going to use before they die, and make sure significant assets that aren't sold are listed in a document in the estate packet.
There's also the option of actually giving things away directly before death, but gift giving is a widely recognized warning sign for suicide, which makes it a risky option.
How do I even do this properly?
There's just no good way, unfortunately. "Less terrible" is as good as it gets, and that takes a lot of work and planning. The fact that your primary concern seems to be the feelings of your loved ones means that you have people who care about you, and you know they're going to be hurt if/when you CTB. There's just no getting around that. Personally, I've kept dragging myself through hell, living for the sake of my loved ones, and I view the work and planning I've put into this as a case of hoping for the best while preparing for the worst... and as a gift for my loved ones. It would be a lot easier on me to just hop on the next bus, but putting in the effort and planning for their sakes is a sacrifice I've made for their benefit. And on a related note, if there's anything you haven't tried that might help reduce your suicidality, do try it for the same reasons, and let your loved ones know. It's harder for them to blame themselves for not making you get treatment if you've been treated for depression unsuccessfully for a long time. (If you go that route, it's safest to talk about being depressed and wanting to die, but NOT admit to being suicidal. You can find the questionnaires used for assessing depression online and learn how they're graded so that you can get treated for depression without risking being committed as a suicide risk.)
No matter what, I definitely recommend putting in the work and planning. It's painful, but enlightening, making you think about things that most people never get a chance to. I've found it helpful in discovering what really matters - didn't change my mind, but I ended up quitting a job that had me working about 100 hours a week, and living off savings to spend that time with my loved ones instead, which has been far more valuable to them than any amount of money I could leave them. Now they have months of good memories and pictures of me that no amount of money could buy. Your circumstances may be different, but it sounds like your loved ones will feel the same way, so put in the work, figure out what matters and spend time with the people that love you. Even if it doesn't change what happens in the end, it's a gift they'll cherish long after you're gone.