From what i understand, you stick them on the car, part will overlap the rear door - cut this bit off, and stick it on the rear door - that's what the proper 5 door ones are like (but they are quite rare)
WHERE DO YOU LIVE M8, MINE'S FLAME RED, WITH TESTAROSSA SIDE SKIRTS ON IT. SHE'S A 5 DOOR ASTRA GTE.
IF YOU'RE INTERESTED, THEY CAME FROM FIBRESPORTS £125
THE ORIGINAL GTE 5 DOOR SIDE SKIRTS ARE NO LONGER MADE, SO UNLESS YOU GET SOME 2 DOOR ONES AND CUT THEM TO FIT, YOU'LL HAVE TO ORDER SOME LIKE I DID. THEY COME IN 4 PARTS, TWO FOR THE SKIRTS AND TWO SMALLER BIT THAT ATTACH ONTO THE REAR DOORS, SO THAT THEY CAN OPEN AND SHUT PROPERLY.
The side skirt part is either rivetted or sel tapped via screws, and the rear door part are glued on. Obviously, you'll want to make sure they're stuck on thoroughly. You'll most likely paint em 1st, as with mine, i had them put on and THEN painted, so in the end i had both sides of the whole car done, thi cost me £300, but if your bodywork's all ok, then just go for the skirts.
Good luck m8,
ps. If you look at Cris Roses car, you'll see the same sideskirts as mine.