I can typically get the shape of masks on paper well enough, either by drawing by hand or on the computer, but getting them transferred to masking tape has always been iffy. Root of this problem is that I can't cut a decent shape with a knife, and any attempts to cut masking tape with scissors tends to end in a sticky ball of curled up tape. I seem to have stumbled on a solution for this though.
First, I draw/print the pattern on fairly heavy paper, and cut them out with scissors. I then stick an area of masking tape on a plastic bag (either a single piece of tape, or several side-by-side for larger areas). I then place the paper version on the tape version, and trace around it with a soft pencil, transferring the design. The combination of bag and tape, by virtue of no longer being sticky on one side, can then again be cut with scissors along the pencil line. Once cut, the tape is peeled from the remains of the bag, and applied to the model (as a side benefit, it has now lost some of it's strength en is thus less likely to tear the underlying paint off when removed).
Currently at war with the Pile of Shame