It really explains it well in that video SteveC linked. You should watch that all the way through first.
I do not like posting these numbers for things I have not tested. These numbers should work for your drivers and your motors. .25v for the Z driver, and .5v for the X and Y drivers if you have heat sinks. After you set these numbers run a continuous test program like a round spiral vase sliced for 3d printing. This will drive all the steppers continuously. You are doing this to check temperatures. If the steppers got hot you need to turn down the drivers a tiny bit. if they get really hot and it stops moving you need to turn them way down. On the other hand if they are cold after running for 10 minutes you can turn them up a tiny bit and get a little more power.
Take your time this is a pretty good thing to learn, you can really optimize your machine right here.