Hi JayX3
I know how your are feeling. I felt the same before I did mine but it all worked out fine and was actually very easy to do. A few pointers that would help, sorry no pictures though.
1) Measure twice, cut once. In my case it was measure 50 times cut once
2) The window is actually a bit smaller than the template on the back of the box. In my research I had found others report that as well so I double checked before I started laying out the cut lines. It is in fact smaller. If you cut to the template it will be OK but you will have some slop of the window in the opening. I wanted it snug so I measured the window inner flange itself and laid out my cut lines based on those measurements. IIRC they were about 1/4" to 1/2" inside the template.
3) I took off the door and worked on blocks in my garage. I felt this was easier than messing around with it upright and attached to the trailer. Much easier to handle.
4) I used painters tape adjacent to and outside of my cut lines to really help mark out the path. This was just in case visibility of my pencil lines became difficult to see while cutting
5) Depending on what you use to cut will dictate how to setup and cut the hole. I ended up using a diamond blade on my angle grinder to cut the main lines and my dremel with spiral attachment for the corners. Probably not the ideal way but it worked well. Main reason I did this way over say a jig saw with fine tooth blade was that I wanted to prevent a rough edge on the exit side of the cut. Also, I tried the jig saw way and I didn't like how it was going. It may have been because of the blade length. Since the door is essentially hollow (just foam in there) if the blade is short enough it bounces of the inside of the under side while cutting as you advance the bade. Which then made the jig saw bounce quite a bit. I suppose a longer blade would fix this but I didn't have one. Doing it the way I did meant I had to cut the opening on both sides of the door instead of just one. So measurement was critical. Upside was basically no clean up needed on the cut edges(filing, etc) although still a bit.
That's about it. Once you work up the courage to just start it is easy from there.