Heres how I do it:
1. Roughen the back of the acrylic letters –
2. cut out a stencil from Oracal 810 and apply to the substrate
Smear SIROFLEX adhesive all over where the letters are to go. You can use a teflon squeegee, and use the thickness of the stencil as a "guide" for the thickness of the adhesive that is laid down.
Press the letters into the spaces. Leave overnight.
When you come back, pull the stencil away, and if you have done it right there wont be any glue visible.
You can also use VHB tape, but the individual pieces show when you look down the side of the letters.