Best to cut one layer at a time.
You can use scissors, but only in the push method on lighter weight dynemma. The scissors will dull rapidly. I wouldn’t suggest them. Instead use a utility knife or x-acto knife where the blades can be replaced. Always use a self healing mat underneath or a surface that you don’t mind cutting.
You can use selvage for single sided tape. You can use your double sided tape for this. However, it doesn’t make great field repair tape due to the long curing time.
I like drawing my cut lines with a pen, or useing painters tape, before cutting on the wrong side of the dynemma. I’ve tried a ruler, but the material is slippery and the ruler position is a battle while cutting.
Hope this helps.