knock yourself out
US /nɑk jərˈsɛlf aʊt/
UK /nɑk jərˈsɛlf aʊt/

1.
fais-toi plaisir, vas-y
used to tell someone that they can do something if they want to, even if you think it is not a good idea or will not be successful
:
•
You want to try to fix the car yourself? Sure, knock yourself out!
Tu veux essayer de réparer la voiture toi-même ? Bien sûr, fais-toi plaisir !
•
If you think you can finish all that work by yourself, knock yourself out.
Si tu penses pouvoir finir tout ce travail tout seul, fais-toi plaisir.