I am gonna copy and paste this from another site, its pretty much the same thing i would say:
A proper box carpet is very stretchy. When you get your carpet, give it a tug, you should be able to easily warp the fabric....this will be important later!.
There are 2 ways to carpet a box. (I'm sure there are more, 2 STANDARD ways) seamless, and seam. I will go through seamless because its always the more desireable way to carpet a box based entirely on aesthetics.
A seamless box is defined by having NO cuts at all, and 5 sides fully carpeted. depending on your box shape, you can use the baffle as the 6th side and actually fully carpet the box! no seams! thats rare to have such a shape however (usually for truck style boxes) so I will go over the standard way to do it.
You can really start on any side you wish, but realize that you have to stretch the carpet. So if you have a box that is 12x6x30 inches, you dont want to carpet a 12x6 panel first! Rule of thumb (and not always always always true) is that you will want to start on the side opposite the side that will not be seen (in our cars, that will most likely be the bottom). So let's say we are carpeting a prefab style boxes for 2 12's.
I would start by spraying adhesive on both the box and the carpet. make sure you spray the right side! Carpet will have 2 side, the face and the backside. You don't want to carpet with the backside facing out!
After waiting as long as the directions tell me to (anywhere from 20 minutes to immediately) I place the top of the box onto the carpet, which is laying belly up on the floor. I'll pick a side, usually the baffle, and spray again, both the carpet and the baffle. I will go extra heavy on the corners, and on the carpet the corners will stick out 45 degrees from the corner, not directly out in either direction.
When I am ready to apply the pieces, I literally stand on the box, and grab one corner. I then tug on it, stretching that section of carpet as much as I possibly can, then I apply it to that edge of the box. Its contact cement, so it should bond instantly. I then do the same to the other side, then i'll stretch the center out a bit, whatever it takes to get in between the sides to have no wrinkles.
Box carpet should be VERY stretchy remember!
From here, it's up to your own discretion whether you do the sides or the back next, it really depends on how much you've stretched the carpet thus far, you can try to lay up the carpet a bit onto the back and sides to see where most of your play is gonna be. Remember you want to have NO creases till you get to the bottom of the box!
Basically you follow the same procedure for the rest of the box, you can spray 2 sides at once if you wish, going a side then half the back, then spraying the other side to get all the adhesive in there, its kinda open at this point!
When you get to the bottom, spray all the way around the MDF and carpet, cut just a couple inches in, you wont be carpeting the whole panel. overspray is just fine.
Then lay on the carpet, at this point, wrinkles and creases aren't TOO critical, so don't break your back over them, but the fact remains the fewer of them you have, the less knife work you have later.
Then simply grab your knife and cut an inch and a half in; so the bottom panel is blank MDF with an inch and a half or so border of carpet. This can be an inch, half in, three inches, it doesnt really matter.
At this point too, you'll cut all your creases on the bottom, no point in having the box not sit exactly flat or anything cause a crease raises one side of the box up a fraction of an inch....
At this point you can cut out the circle, too, just hold the blade perfectly flush to the hole and run it in a circle, if you have a super sharp blade (you know those boxes with like 100 disposable blades in them?) it'll cut it like butter and you'll be set.
Done! Your box has been carpeted. Or vinyled.
EDIT: when it refers to our cars, its referring to a 7th gen civic (01-04)
