I found cutting 6" strips, the lenght of the roll (I just cut it all freehand as exact size doesnt matter) works great for the inside of the STi / WRX doors. I usually make a few wider pieces and a few thinner ones too. You can get away with bigger pieces in the middle of the door, and u might need a smaller piece for the bottom. That way you can put the strip up inside the door and get an idea of where you will be laying it BEFORE you peel off the backing. At least thats how I did it. If you have big arms you might have some trouble getting them up in the doors.
You can just cut the stuff any way you want to and theres really no WRONG way to do it (unless you stick it to something that needs to move such as the door handle and lock rods. You dont want to bind them up at all. Same goes with the window on the inside of the door.)
I found it really easy to pull off the plastic rain shield inside the door (WITHOUT DESTROYING IT) Then take that and lay it on top of my roll of mat and make a full copy of it including the cutout for the door handle, and the 2 slices where the stock wires pass thru. This allowed me to make a single piece that covered the entire door with ease. You just pull the wires thru the mat, and slap it on. (this was possible alone, but would be MUCH easier with a spare set of hands to hold the mat from sticking to everything it touches.
Once you get that huge piece on the door you need to cutout the speaker hole. I cut mine in a star like pattern sorta like ~> * but with a few more lines in it. Then without cutting the pieces off I took them and pushed them inside the door and stuck them on the backside of the panel. if that makes sense
When doing the outer door skin (interior side) you really need to decied what you goal is before you begin. Rick already laid out the options, you just need to pick one.
As i stated before if your in a cold environment cut your pieces before you plan on working and leave them in your boiler room. Also you might want to get a high powered hair dryer, or a heat gun (Be cautious with heat gun as you can actually bubble the paint off the other side of the sheetmetal if your not careful) I just use a high powered hairdryer and it works perfectly.
I will try to get some pics on the next car I do. I dont think it will take that long seeing I just matted the doors on another STi last night (3 in the last 3 months now?). I guess this is going to be the new cool kid mod
Its really amazing how much of a difference it makes on an STi. It is clear as day the car has been matted just by the FIRST POP when you pull the door handle. Nevermind from the way it shuts etc...
If I ever aquire a video camera (and a stock STi) I will make some videos of the difference