i use a light table, but should be able to do w/o one:
put your letter face-down on table with pattern face-down on top of it
shift til letter matches drawn letter shape, use finger to feel where the hole is and a pencil to punch-thru the paper into the hole.
repeat for all letters, then flip pattern-over and circle the holes with a marker to help spot your holes quickly during install.
otherwise, our shop guys are still keen on just aligning letters with studs on pattern by-hand and pressing-down to indent the paper & circle those marks with a pencil.
......and then there's the obviously awesome solution:
if making your own letters, route face-down and drill your mounting holes before your cut your letter. then you can simply plot your hole pattern and skip all the nonsense above.
if welded-studs, then I route my metal face-down and engrave 3/16" circles for my shop guys to have targets to hit their welded studs to.