Warning - the mounting holes/webbing inside the cutout are not perfectly centered. The window cutout is fine, and this just means that the inside guide will only fit in one orientation. It's such a non-issue that it wasn't worth fixing the model, but may confuse you at first.
To use :
1- Place the guide against the back wall of the cab. Hold firmly and drill 4 mounting holes.
2- Place the small guide inside the cab and secure both guides, with body shell sandwiched between, using M3 screws and nuts.
3- Drill out most of the material inside the hole.
4- Using a hobby saw or other tool, carefully cut the outside edges of the hole while following the guide closely.
5- Using a file or sandpaper, sand the edges of the cut down to meet the guide.
6- Remove guide pieces and cut remaining 2mm tabs.
7- Sand tabs flush with the rest of your cut.
8- Carefully paint edges of hole (or just use a sharpie like I did)
9- Clean up clingy styrene dust for the next 2 weeks.
The author marked this model as their own original creation.