My first attempt didn't turn out so great:
I usually glitter the letters in this way (with glitter glue pens) and it looks okay, but it didn't work on white lettering. I tried using regulr glitter and glue on top and that didn't work either.
My next idea was to create the letters with gems. Great in theory, but didn't look too great in practise:
My third idea was to print the letters onto thin card (Arial black font in 100+ font size in white with a black background/highlighter behind to see where the edges of the white letters are). I cut these out and got hubby to spray paint them silver:
These were easy and turned out as cleanly shaped and coloured letters in the size I wanted.