Custom Hand Setup Tutorial (Old Method)
To use your own hand models, you will want to make sure that you have a pair of hands where one was mirrored from the other in the 3d modeling program. This will allow the hand to be mirrored rapidly in the hand poser editor.
If you have a single hand model you can follow this guide created by a fellow community user to created the second hand.
Hand Setup Tutorial
This process has been replaced by the helper wizard here Hand Setup Wizard. It may be useful if your hand rig isn't mirrorable using vector math, or if you aren't able to get the wizard to work for your hand structure.