1.take everything off the new a deck
2.under the folding mech on the b/pro, there is a little metal plate with 4 holes in it, put that on top of the new a. use the 2 stripes that go down the deck as a guide to get it centered.
3. use a sharpie or something similar to mark where the 4 holes are (these 4 holes are where the bolts go to hold the folding mech to the deck)
4. this is where you have a choice, and i think where probably a lot of people fuck it up.
k, remember that weird 3 sided plate you took out of the new a deck? you can either use that, or the plate you used to trace the holes, the only difference is, the plate you used for the holes doesnt quite fit right, so the edges will bend if the bolts are tight, but it still works. OPTION 2: you can use the new a plate and its a logical idea since it fits in the deck better and will be tighter, BUT you have to cut pieces off the sides of the plate so the heads of the bolts fit.
5.k, take whichever plate your using and put it in the deck. if its the b plate you wont have to drill any holes, if you use the new a one, just put it in the dekc and drill down through the holes you already made.
6. now your going to have to drill 2 holes in the bottom of the deck slightly bigger than the head of the bolts so you can put them in (the 2nd set of bolts, closer to the back) note: MAKE SURE NOT TO DRILL INTO THE SIDE OF THE DECK, or it will be really weak there when you go to ride it and will probably break.
7. put the folding mech on top of the deck, plate inside the deck underneath it, and screw each bolt in until they are all tight. the new a plate will fit real nice and the bolts will be tight without much work, the b plate will bend and it wont be pretty, but it works.
the end.
if you dont understand any of the parts i can probably explain better/use pictures or something