How To Line Up Number Decals?

I usually line them up along a panel line. There is usually one in the appropriate location.

I have sometimes stuck a Post-it paper just below where the number will go.  Those 3m Note papers have very low tack which will generally not remove previously applied decals as long as they are thoroughly dry.  To be extra safe, I generally apply any numbers of that type as the first decal(s) just so there is no danger hurting the other decals. Post-its will definitely not take off paints or leave residue.