What you were told is not quite true.
"Other income" does go on line 21 of form 1040. "Other income" is anything that doesn't fit into one of the other assigned income categories. Most "extra" income will fit into one of those categories. You need to better describe the source of this extra income, to determine how it should be entered on your tax return.
The IRS considers undocumented cash
income (no W-2 or 1099-MISC), for work performed, to be self employment income.
Enter at "Business Income & Expenses" and TurboTax (TT) will
complete Schedule C or C-EZ for you and allow you to deduct any expenses
associated with this income. You'll also have to pay self employment tax
(social security) on any profits greater than $400.