You report the income from your own records. The IRS considers undocumented cash income (no W-2, 1099-K or 1099-MISC), for work performed, to be self employment income. Enter at "Business Income & Expenses" and TurboTax (TT) will complete Schedule C for you and allow you to deduct any expenses associated with this income. You'll also have to pay self employment tax (social security & medicare tax) on any profits greater than $400.
You report the income from your own records, no matter how weak those records are and even if they consist of nothing more than your memory. If you plan to claim refundable credits (earned Income Credit, Additional Child Tax Credit), based on this earned income, there's a chance of some kickback from the IRS. The self employed are expected to keep records. Getting paid by check is best. Direct deposits or cash payments in a bank account are common ways of creating a paper trail.