3D designs

Your design needs to be digitized speacially for foam. It should contain appropiate settings so that foam would show well.

A good and professional digitizer would be able to do 3d/puff digitizing as he would be experienced and have technical knowledge.

First you must do some run stitches across the design area in order to tack down foam(prevent foam from moving along sides).
After that,everything depends on type and shape of design or letters you doing.Ussualy on most cases edges should be covered with stitches in opposit direction in order to prevent excess foam to "jump" out.
Your density must be higher(0.20-0.25 mm),and again underlay is dependable on design shape...on smal areas underlay is no needed or just simple zig-zag stitches will be enough.
When covering large areas,double zig-zag with high density(0.80-1.00mm)will be more aprociated.

Small details are sometimes unpossible to do with foam,so in that cases higher density with more underlay will do the job.

Good Luck!

What Bob1 said has pretty much covered everything you need to know how to create 3D text.

Here are the pictures that might help, good luck !!

http://centdigitizer.com/3Deffect.JPG
http://centdigitizer.com/foam.jpg
http://centdigitizer.com/caps.jpg