Hi Sophie, are you rooting hardwood cutings or softwood cuttings? For hardwood cuttings, I use slightly damp, not wet paper towel inside ziplock bag.I open the bag and check every day. That helps to "air" out the mold somewhat. If there is mold, just take clippers and snip the moldy part only, if it's along the edges. For softwood, I use Bass's technique that on his Trees of Joy blog section. Instead of using black pot, I use clear plastic cup . And I place it inside of a 2 liter soda bottle, that's cut in half. You leave the cap off for ventilation.It should root with minimal watering. For soil, I use 1 bag of Miracle Grow Seedling mix with 2 bags of perlite, all mixed. Hope that helps.