I would highly recommend doing a sample block first for a few reasons:
you want to make sure your choosen fabrics will "bloom"
you want to make sure you like the look
you want to make sure you like the time involved before venturing forth onto a large piece.
Fabric choice: cottons, rayons, cotton/rayon blends
Linen, wool, and silk are also mentioned in the book as possibilities but I haven't experimented with these yet.
Looser wovens work well.
Do NOT prewash.
Yarn dyed and other fabrics with good dye saturation give better color results.
Your prints do not have to match- you just have to like the way the colors go together.
Polyester, acetate, etc usually won't work.
For the sample- cut out a square or rectangle on the straight of grain in your base fabric- mine was roughly 10x12. Your base is the fabric that won't get slashed. It can go right side up or down depending on what you are making. But more on that later.
Your stack: also cut on the straight of grain. I used 4 layers of cream colored 100% cotton sheeting in my sample. Boring I know but that was all I had that wasn't prewashed. I *would* have gone to JoAnn's and bought more fabric (just for you guys

) but no car until Sunday...
Rule of thumb for # of layers 3-5 for cottons, up to 6 for rayons. Rayon will "bloom" better usually. Cut your layers about an inch all around smaller than the base. You will get a different look depending on the order you stack if you are using different prints. You can make up a few samples trying out the different order if you are feeling gung-ho.
You can add an "applique" to the top. Choose something with simple lines. The edges will get blurred and may change shape a bit after slashing and washing. Make sure to cut it on the straight of grain. Choose bold contrasting colors because the colors tend to fade. If you start with pastels they may appear very washed out in the end even if they have ok contrast now.
So get stacking and I'll add further instructions and photos in the replies.
LMK if I left anything out or it isn't clear.