I have 1 main method for cutting angles:
The main thing is that I have a fairly good judge of the angle itself so I cut it without any direct measurements but you could do measured cutting. "freehand" but I will give my method a shot. and for your purposes, it may be easier to cut the "flat" pieces first and then merge the more angled pieces together at the angled edge.
but first thing first. you need to know where your long edge is going to be.
* Mark the long edge appropriately along the length of the cut. (the long edge should often be the clean or detailed side of your cut. if cutting away from the detail flip these steps.)
* if doing a measured cut measure your angle and mark the short edge of the cut. (like with the image)
* from here I use a 25 mm snap-edge blade to make my cut. (the 25 mm is the largest of the 3 main snap off blades you can get and is often plenty long enough to make most cuts with ease. [note of caution: these blades are exceptionally sharp if you are not careful it is very easy to cut yourself])
* when making the cut begin by lining the length of the blade with the long edge (detail edge) and very gently glide the blade back and fourth until the blade has caught the foam. (the very edge of the blade should lightly mark the foam so if you lift the blade away a thin blade line should be present.)
*From here you should be able to match the angle of the cut to the short edge (garbage edge) by either following the previous step with the side of the foam or by eyeballing the angle of the cut with the snap blade.
*again gently move the blade back and forth while applying consistent downward pressure for the length of the cut while maintaining the required angle using the length of the blade to cover the full distance of the cut. (once you begin an angle cut try not to pull the blade out or "backtrack" to make corrections as this will often create a cut line in your foam. I still make this mistake so no worries at the end of the day. and if not doing a detail side cut then cut lines are not important anyway.)