Here's what I've done for lettering:
1. Type out exactly what you want, in the correct size, on your computer and print it out
2. If you have decal paper that's letter size, most good copiers can copy onto that. Copy the printing onto that copy paper
3. Then, hit it with a coat of dullcoat or glosscoat
Put the decal in place like you would with any other decal
I've had good results with this method, for vehicles and rolling stock when I'm doing simple black lettering...
Commanding Officer, 796th Railway Operating Battalion (in On30 gauge), operating on the Stoney Creek branch of the ET&WNC RR