If I were considering jumping as a method, then I'd want to :
> find somewhere that's 300 feet ( 20 floors ) or higher.
> be certain I'll fall without interruption or landing on a ledge, etc.
> make sure I hit something hard, and that nothing could break my fall.
> avoid windy days ideally, since that could interfere.
> ensure I don't put anyone else in danger, e.g. pick a time when nobody is in the area.
With any method, you should do as much research as you can (eg try the search and advanced search on this site, and put the time and effort in).
I've heard some people suggest the use of aspirin or warfarin, for blood thinning, so in case you didn't die on impact, then bleeding should complete the process.
If you do some good research and gather some valuable info then consider posting a thread, so others can benefit from your research.
Note : As always, please note that I strongly encourage you to seek help with whatever is troubling you. With the right help things can improve. There are many sources of help if you look and reach out. Please seek assistance in any and every way possible. You alone must decide upon your actions and take full responsibility for them, and you should always favour seeking help and keeping out of harm's way.