Roth Ira calculation

I'm self employed person whose income consists of business income, SS, and RMD. I know when calculating my earned income SS and RMD don't count. I've been trying to figure out how much, if any, I can contribute to my Roth IRA. I tried using Form 590A, but, that form is the densest form I've ever tried to use. I gave up after several hours and a headache.  Is there some rule of thumb I can use to try to figure out if I can contribute and, if so, how much?  I've already completed and submitted my taxes for 2021. So, I don't even know if I can make a contribution assuming I'm eligible to even make one. I would appreciate anyone's comments. Thanks