1) Practice on some scrap doing corners and such with the different temps.

2) Pick up a 100 pack of razors for ~$7 at a big box hardware store(Home Depot), that way you won't mind tossing a blade when it starts to dull. I went through about 10-20 in my build. (Cover those blades you toss with a piece of scotch tape so you won't have to worry about getting cut while going through your trash bag or whatnot later on.

3) Imagine what the end result will look like and cut your pieces accordingly...