I just moved to the US. In my home country, everyone give tips in salon or spa by putting the money directly in the professionals’ hands or pockets, is this appropriate in the US? Or is it better to tell the cashier that you’d like give A or B certain amount as tips? What do you usually do?

I have a gift certificate for multiple treatments at a spa (massage, facial etc…)

Do I tip each person seperately after the service or do I tip one person at the end. How does that work?