I believe the process/qualifications vary from state to state. I was qualified for Disability in 90 days, on the first try (I know of people with documented physical disabilities that have been denied numerous times); I hear when one has mental disabilities---in my case barely able to remember the simplest things, etc. (I was given a Psychological Exam through the SSI, and I guess I "passed."), it's easier to be approved because when one is mentally impaired/disabled they can't hold down a job as easily as someone who maybe just can't stand for long period of time... does that make sense?
Having a long job record helps a lot--I changed jobs a lot over one year's period, though as long as you've been employed for many years cumulatively, your chances of your claim being accepted are higher.
I applied for the "initial/general" info on the Social Security website, then they sent me a bunch of paperwork to fill out, then the Psych exam, etc. I don't know if this is available for many states/regions, but the Mental Health Center here recommended a Disabilities Advocate---someone who is not a lawyer, but who, if you're denied benefits, will advise and "advocate" on your behalf.....if they get your next attempt to go through (which happens 90% of time), they only get 25% of your total monetary amount/backpay (which is considerably cheaper than using a lawyer). Thankfully, my claim went through without needing the guy's services, but he was exceptionally helpful during the process anyway, providing info/answers, free of charge (lots of emailing, he was very forthcoming). I was so grateful to be referred to him; I hope you can find someone like that....it takes some of the load of the process off you.
I'll be honest; I found the filing process for SSI long and very tedious. I definitely needed help filling everything out, as at the time I was on a med with a very sedative effect.
If you have any more questions, feel free to message me. As everyone else has said, doing your homework, reading as much as you can helps to prepare you for the process.
Good luck!