When you work for yourself, nobody enrolls you in a retirement plan. There is no HR portal, no payroll deduction and no company match. You choose the account, open it and decide how much goes in.
The good news is that there is more than one type of plan built for this situation. A recent NerdWallet guide, 5 Self-Employed Retirement Plans to Consider, rounds up five options, including a SEP and a Solo 401(k). This article does not repeat that list. It gives you a way to compare plans so the details you read make sense.