I have seen two kinds of crazy quilts,
1) where the fabric is totally random sewen onto a muslin background
2) where the block is pieced using the same shapes in each block, which gives a sort of random look to it when you use many different fabrics.
The other alternative to this is to use the same fabrics for each of the same pieces in the block for a much more controlled look.
All are very effective in their own way.
Gal