While one must plan, do not overthink this. It is an ancient method about as far from rocket science as we can get.
I disagree. There IS a high likelihood of success, but lack of success can easily lead to permanent brain damage and disability. Whatever hell you are living now will be unendingly worse.
The rope or cord must be of proper tensile strength. Depending on partial or full suspension (or drop if you are going that route), you want to be safe and have your body weight be a fraction of the rope's minimum breaking weight. For some rope materials, this could be 5-20% of the breaking weight would be equal to a safe working load. All that any of this means is, you don't want the rope to break, which happens a lot and people fail.
In addition, the proper knots should be used. Climbing knots (for rock climbing) or other hitches for boat rigging or other industrial practices are good options to secure to the anchor point. Most people don't know this, but knots weaken a rope's tensile strength by up to 50%. Not usually a problem, but if you have a rope that is thin or already close to breaking points, the breaking point will be at the knot. This is because tying a knot bends and stretches the fibers at the knot point. What does this mean? Two things. One, if you tie the wrong type of knot on the wrong type of rope and you are too heavy, the rope will break at the knot. Two, if you don't tie a proper knot (it's not the same as tying your shoes or just any knot) it can come undone prematurely, leading to a failed hanging and brain damage.
These are only a few considerations. Rope material is important to avoid stretch as this can result in your feet touching the floor when the system is engaged and you kick out the chair from underneath you. A typical noose is also not the correct knot for hanging. That is for a drop hanging like you see in the movies, or the wild west. Judicial hangings. It may work, but it may be less than optimal, may not slide well, and if you are too light, it may result in a hanging that takes a drastically longer than necessary amount of time because the noose did not fully tighten down. Lubrication of the rope with beeswax or something else (depending on the type of rope) at the sliding point can be important.
Just playing devil's advocate. There have been books written about this matter. Militaries have written entire guides about how to conduct a proper hanging including rope thickness, knot types, structural integrity of the frame from which the rope will hang, and more.
That being said, tons of people who attempt hanging can get it done with a bedsheet, or shoelaces, or even a t shirt in prison.
My only point is that tons of people also fail because they didn't do it right.
I wouldn't test my luck with any of that. You'd hope to have a strong, correct rope, the right knots, a proper anchor point, and all the other variables.
People have been successfully hanging themselves for centuries, but they've also been fucking it up for centuries too.
If you dig up the statistics, the vast majority of suicide attempts are failures. We see a few on here all the time.