My first question is: really? Are you absolutely sure? Positive? Can’t just use some hair product or a hat?
Ok, if you are still determined to do the deed, then consider reading this: How to Cut Your Own Hair at Home (Long, Short, Wavy, Curly, Kids, Bangs) | WIRED.
Despite being in Wired, it is surprising detailed and gives lots of practical advice.
Good luck. And if you are using anything electric to cut or trim, make sure it is well charged and not old: you don’t want them giving out on you midcut.