For fish that are over night shipped I contact the supplier and match the salinity of my QT to theirs. I then float the bag for temp acclimation and go straight to QT. An interesting situation yesterday was fish from two different suppliers coming in at the same time. One (Diver's Den) ships at 1.025 and the other shipped at 1.022. I set my QT to 1.022. Going from higher salinity to lower is much safer than the other way.
For local fish that haven't been in a bag for 18-24 hours I do a regular drip acclimation like sabella said.
I don't qt fish. You can qt a tang in a small tank give him ick cure it. And put him in the dt and he can get it again. It's like a cold sore for humans it is natural if you don't feed properly. If you don't know this you haven't been in it long enough.