How I usually do it:

1. Cut out the decal so that it would fit nicely on the plastic piece.
2. Secure it so it does not move around, with either modelling tape or if I'm lazy just hold it down tight.
3. I use a pencil to rub the decal onto the plastic. Preferably a dark 2B pencil so you can see the parts you've already went through and not miss and part of the decal.

May not be the best way to do it but no problems for me : )