Using decals like anything requires practice and patience. Breaking the decal up into smaller pieces rather than using it in one go will ensure a straight line once you master it. The term dark art gets thrown around a lot when describing some skills involved in this hobby and if you see using a lining pen as beyond your skill level then that's your problem unfortunately and there is nothing anyone can do for you as you have to want it to own it. The great drummer Carl Palmer once said that he could teach anyone who had never sat behind a drum kit to be capable of drumming on a record of a top recording artist in six months, all it takes is practice, honest effort, and desire. If you haven't any of those qualities than as I said you have a problem.
Rich,