I would not cut up the moss until you have seen it growing. Use a tray that is at least 2 to 3 inches deep and pour in the pumice and lay the moss on top. Water with either distilled water or reverse osmosis water. No tap water. No fertilizer. Be sure the tray has holes in the bottom or along the sides or use an ice pick and make them so the water will drain through. I would let it only get 4 hours of direct sunlight and no more. Probably best to water heavy in the morning before the sun falls on it. And even at night especially if you get drying winds. Good luck.

Moss tends (for me) to be hard to transplant. Just a comment. Good luck.