how to enter self employed tips?

They have to be reported on a 1099 in order to deduct them.   If they are reported on 1099, TurboTax will ask you when you enter the 1099, whether or not any of the income is tips or overtime.  How to enter self-employment income.   You won't be asked about tips or overtime if your self-employed income comes from only cash or checks (non-1099 income).  

Can someone please reference IRS rule/regulation that states it "HAS TO BE ON A 1099 form.  I am not able to location this information.  I do taxes for a Salon/Hairdresser.

If the hairdresser is employed by a salon, they can use Form 4137 to report tips, but it still has to be linked to their W-2 or 1099 form reporting income.  A self-employed hairdresser can report income received on a 1099-K with a personal record of tips to report tips.  Here's more detailed info from the IRS.

@Raphnessnt 

You can enter the tips but the deduction does not lower the taxable income??

When you are entering your income for self employment, TurboTax will ask if any of that income will includes tips. The tips must be included in a Form 1099-k, 1099-MISC, or 1099-NEC.  You have to keep logs that show the date, the customer, and tip received.  

Guidance for Individual Taxpayers who received Qualified Tips or Qualified Overtime Compensation in 2025