Masking the canopy, particularly on an aircraft like the Avenger, is a tedious process. It is also the part that I typically dread on any build.
Having said that, what I recommend is a new Xacto knife blade, a good cutting surface with a metal straight edge, and if your eyes require, a good Optivisor and a good source of light. I typically start cutting strips of Tamiya tape and affix along a frame. I will usually work along the boundary as defined by the "frames" then fill in the gaps. Working one "pane of glass" at a time.
There are other ways of doing these kind of canopies, so you may do a search on here to find alternative methods.
The kit is an old one but can be built into a good-looking Avenger, but the Accurate Miniatures kit is superior. Good luck on your build.