I'd imagine you're also including something to cover your nose with as well?
I don't believe self inflicted suffocation is a smart way to catch the bus for these reasons.
1- Hard to make that truly airtight using that method you described. Even with a plastic bag tied around your neck, your natural drive to live is gonna try to rip it off pretty quickly.
2- If you're planning on tying up your hands to stop that from happening, good luck with that, but you're probably not gonna make it anywhere near tight enough without enough oxygen in your brain. You'd need someone else to tie them up for you.
3- It's gonna be a HELL of a way to go. Trust me. I've tried doing similar methods as a test, and its beyond excruciating. You're not gonna pass out peacefully, even if you are able to get past your basic survival instincts, which will be your main obstacle ofc, you won't loose consciousness without an awful fight. You'll probably pass out within a minute or two if you aren't a swimmer or something, but getting to the point will be the worst pain imaginable.
4- If you fuck it up, vegetable status is pretty much confirmed. And its really easy to fuck up for all the reasons above.