With the help of an advocate at the clinic I go to. That lady knew exactly what to do, say, fill out forms and when to expect things to happen. I suggest looking for someone like that, maybe at your local senior center, even if you aren't a senior. For me it was, in some ways, easy since I had her to lean on. The hard part was living in my car for ten months waiting to be approved.
So the advocate lady helped with the online application and we called the nearest Social Security office to ask questions. It's pretty complex so having help is worth seeking out.
After that you will be given an interview date, which can take place over the phone. Once you have the interview, it generally takes 90 days for a response, meanwhile you may receive additional forms to fill out and they could come from your state instead of SS.
You absolutely have to have medical backing and proof like MRIs or whatever shows your condition. They may not look at it until after the 90 day waiting period.
At the end of 90 days you will get a rejection notice. Immediately request a medical review and make sure all doctors who can affirm your condition. This starts another waiting period of a minimum of six months after which you will be contacted again.
Many have to go to court after this point, which is when you start to consider getting legal representation although I know of some who represented themselves and won their case. I was fortunate in that, since I was homeless, my case was expedited. It took exactly 90 days for the medical review, they called me on a Saturday to inform me that I had been approved since the MRIs made my case for me.
Note: You cannot work or have any kind of income during this period, have to be out of work for at least a year when all is said and done, which is how I ended up in my car. It's hard to get through the process but once you're there, it's better.
Best to look around for someone to help with the application, the lady who helped me also does insurance counseling at the clinic.
Good luck!