I'm a noob to fixing things but i want to fix my car's AC, it's not blowing cold air.

I already had it topped up at a shop and i could hear it hissing when i turned it on, it was empty again within 24 hours and when i went back for some reason they bullshitted me and didnt check with a UV light for the leak or anything.

I want to learn do it myself, i'll order one of those kits with gauges and a vacuum pump and cans with refrigerant. I already thought about trying to change the valves in case thats where it leaks from, but if its somewhere else, can i just load some dye into the system somehow and then check with a uv flashlight to see where its leaking? Or do i have to put 500gr of premixed refrigerant and let it leak again, cause that would be pricey.

Whats the best way to go about this? Thanks.