Thanks Pantha! I've built a few 89 suits for myself, over the years and have done it various ways, including wearing the suit while the rubber armor was applied. In fact, this last build was done that way because my mannequin was destroyed.
The easiest way, is to have a friend help you.
Wear the under suit but if your under suit is not coated with a stretch vinyl surface, you may want to put on a thin expendable garment underneath the under suit, to protect your skin and hairs, from getting glued with the rubber armor, through the fabric. Happens a lot.
Wearing the undersuit, is the next best thing, aside from having a semi-rigid foam copy of your body to work with. But again, you will need a friend to help you line up and hold the rubber in place, until the glue reaches minimum adhesion, before you can pull it off gently and go back to reinforce the bonded areas, in small sections.
One more thing. Don't glue the armor on too tight. Leave some leg room for mobility. Some have glued on their legs and torso, so tight, that it became an issue, once they wore it. Making it hard to get in and out of and you will start to feel like your body is suffocating. If you're not claustrophobic, you will be after a bad experience like this. (I've learned the hard way myself).
If you have wrap around leg armor, leave decent enough space, in between the over lapping rubber, so you have enough spandex (or whatever material under suit you use) to help stretch while you walk or sit, etc. Don't glue around the collar either. Leave that open, so air can go in and out. Other wise, it will create a suction, every time you breath in. It's a horrible feeling. I would even recommend cutting out 2 or 3 holes, the size of dimes, around your under suit's chest cavity, where the armor will cover up. Doing so, created a good ventilation system, and removed that nasty suction effect.
So, when gluing on your suit...
1. wear the cowl first. Keep your head in a comfortable position. Otherwise, you will have to surrender to the position of the chest armor and if you glued it too high or low, it will then dictate how you keep your head in position, which may end up being not so comfortable.
2. as you stand with the cowl and under suit on, you start by slipping on the torso armor and line it up to the oval on the cowl, again, making sure your head is in a comfortable position. Once you get that down, have your buddy glue around the torso and arms. Making sure the arms are not glued on tight either, for the same reasons I mentioned above. Just a good gentle wrap around your arm, is all that is required.
3. then comes the leg armor. If possible, do this in front of a mirror, so you can see how high or low, you want the leg armor to sit around your waist.
4. at this point, you can remove the armor, gently, once the adhesion has reached it's minimum strength (not maximum, that would take at least 24 hrs lol). As you pull it off, you might detach bits of your armor, but no worries. As long as the rest is still bonded in position, you can freely reinforce it over a table, with towels stuffed in the suit, to help you out.
5. the arm, calves and knee cap pieces, can all easily be done by yourself, simply slipping in one leg at a time and one arm at a time as you work along.
I hope this helps. If you have anymore questions, I'll be happy to help. I may be a bit caught up with work and may not always respond right away, but I'll make sure to pop in when I can.