Something you might consider: If i I were in your situation and if fiberglass isnt that expensive, I would practice making other junk first just to get used to it. That way, when you actually do your dash it wont look like poo.

 

i could practice by building a rear tail panal blackout

 

OOOOOPS. Double Post. Please disregard
You will cut, sand, shape the foam to the almost exact size and shape that you want the finished part to be. The layers of glass will add approximately 1/8 inch. thickness to the foam mold. Be sure to make provisions for attaching it to the dash and areas for the guages to mount to. If the mold has tight compound curves then laying the glass cloth (not matt) will be difficult. You may have to cut the glass in several pieces to get it to form around the curves. If the mold has rather large radiuses you might be able to use just one piece of cloth. When it is cured, you will remove the foam from the backside (if it is not too complex of a shape) leaving a glass form of the desired shape.