The issue is that if you paint the suit as its lying on the floor or table, and then put the suit on, the stretch of the suit will distort your work. If the paint bleeds through, too, it can cause a real mess.
The best way to tackle this approach is to get a friend whose artwork you trust, OR a friend who has very similar size/weight/dimensions as you. You (or similar friend) put the suit on, and then your artist friend (or you) handpaint the designs while the suit is stretched to its wearing dimensions. Whoever is wearing the suit will need to hold still, and not move around, sit, or brush up against anything until the paint dries, so you may need to do this in stages, based on how detailed the design is. This method should prevent the paint from cracking when you put it on again later.
Some fabric paints have a tendancy to stick to themselves or other surfaces even when dry. Just be aware of this, and test out your paint first.
Pictures and tutorials from my costuming portfolio at Syagria's Cosplay Site!
Star Trek Original Series Uniform!
Original Creation for AB13
Most Recent Con:
PAX East, Anime Boston 2013