1) Your pattern that you are building on, the pattern that you have cut out, and especially the glass you have traced the pattern onto must ALL be numbered or the numbers are pointless.
2) If you are using tracing paper to make your copy onto poster board, number the pattern while the tracing paper is is still in place this way you only have to number things once. This also helps prevent numbering errors and saves a GREAT deal of time.
3) It's easy to miss a piece. Don't worry. If that happens look number it the same as a piece next to it and add the letter B to it. 38B for example.
4) DON'T FORGET TO NUMBER THE GLASS YOU HAVE TRACED YOUR PATTERN PIECE ONTO! I see this happen all the time where the pattern and the poster board pattern have numbers but not the glass itself. The purpose of the numbers is to help you place your glass on the pattern. If you haven't numbered the glass after you traced around the poster board pattern onto it you've numbered your pattern for nothing.
And that's it. Happy numbering!