Hey Tvar,
Mounting is one of the hardest (and usually most overlooked) parts of assembling one of these suits. Here's what I'd recommend:
1) Start by foam-filling the larger muscles. Depending on where you got the suit, the shoulders can be very, VERY large... which will lead to the armor warping or bubbling whenever you move in it. The best way to prevent this is by filling the suit with foam padding.
2) Dremel about 1" of every edge on the back to make sure you have a rough surface to adhere glue to. Urethane can peel from many glues, so having this area "scuffed up" will help it stay down. You might also want to do this under the pecs, across the center of the abs... anywhere where the urethane will really sit flush against the neoprene. Just be careful while you're doing this, it's real easy to accidentally dremel holes or to scuff the front if you're not paying attention.
3) Buy a gallon of contact cement. If you can, get the stuff that comes in a red can - the green stuff is less toxic, which unfortunately means it doesn't stick down as well. I'd advise avoiding the "gel" version too, but that's just me. While you're out, pick up a box of chip brushes, because you'll need a lot.
4) FOLLOW THE DIRECTIONS OF THE GLUE. Don't get hasty, you've waited a damn long time to do this all, so don't borf it up by being hasty... If I remember right, you coat both sides, wait 10 minutes, coat one side, wait 5, and then stick the two together. Go slow, make sure everything's touching, and then press down as hard as you can. I knew a guy who used to use a rubber mallet to smack stuff together to promote adhesion... not the best, but you get the idea. You can do it in chunks... I used to do the main chest and abs, then one shoulder, then the other, then one arm, then the other, then one shoulder blade, then the other. Slow but steady!
5) Go back with loctite or a similar superglue to seal any loose edges. It's easy enough to remove any extra contact cement, but it's impossible to remove any superglue from neoprene, so again, go slow, and think small. It'll dry fast, so you don't need to be doing 6" chunks at a time.
6) Then repeat for every. single. part. The legs, the cod, if you have forearms or calfs... everything gets done the same way. Thankfully you can dremel the entire backs of the fins, toecaps, and boot birds (unless you're using captains or similar boots, then you'll only sand about half). I'd also recommend backing your belt with neoprene, otherwise it'll eventually scuff up the armor around the waist from rubbing.
7) And after all that there's just... I guess attaching the cape clips, adding the lockdowns for the cowl, and getting a cape? Thankfully the cape doesn't close in the front, or attach to an emblem, but I don't know anybody who makes the Forever/B&R style capes anymore. If you find anyone, let me know!
I hope the costume turns out really really well, all the best of luck!