For side gigs, you file a schedule C. You are required to keep accurate business records and report all your expenses and income, even if you don't get a 1099 from all your clients or customers. Your "ordinary and necessary" expenses are subtracted from your income, and the resulting "net income" or profit, flows to the form 1040 where it is combined with your other income, deductions and credits to calculate your income tax. The net income also goes to schedule C to calculate 15% self employment tax.
So as far as estimated payments are concerned, if you expect $5000 in net profit, your estimated payments should be 30% (15% income tax bracket plus 15% SE tax), or 37% (22% income tax plus 15% SE tax) or possibly higher.
