Repairing AC leaks usually means replacing the leaking part or seal. This could be the evaporator, the condenser, the compressor, or the lines, or perhaps an O-ring has gone bad. The majority of small AC leaks are at the service fittings. Especially after service, these fittings tend to leak. If you have a slow leak that's difficult to find, I'd suggest taking a good look at the service fittings. You can sometimes
replace just the valve, but other times you might need to purchase the entire AC line in order to repair a leak at one of these fittings.
When repairing one of these leaks, it's very important you do it properly. DO NOT VENT REFRIGERANT INTO THE ATMOSPHERE. This is very harmful to the environment and illegal in many locations. It's one thing if it leaks out; it's another thing altogether if you intentionally cause a leak to happen during AC service. Before servicing an AC system, you need to recover any residual refrigerant remaining inside the AC system before opening it up. There are special recovery machines that do this work.
As stated above, if you're going DIY, you might consider taking your vehicle to a shop to have the old refrigerant evacuated before you start your repair. After you've completed your repair, you can then take the vehicle back to have the system vacuumed down and recharged.