Okay, here's what you do ...
Paper buckles because the rag (cotton - most good watercolor papers are 100% rag) soaks the paper and expands. Most papers are sized, but this doesn't stop it from happening when the paper has loads of water put onto it. There's a few things you can do to stop this:
1) Soaking. A good many artists buy WC paper in standard, full sheets (20"x30") and portion it smaller, but before they do they get a big bin (big enough that the paper can lay COMPLETELY FLAT) in the bottom of it. They put the sheet in and let it sit for a few hours in water that completely covers the paper. Depending on the poundage of the paper, you change the soak time. You don't want to soak a 90# paper near as long as you want to soak a #300+ pound paper.
When the paper is completely sopping wet, CAREFULLY remove it and immediately fix it to a sturdy board. Do not use regular foam core/board, it will warp and you will be unhappy. There's a special kind that is VERY popular with make WC artists called 'gator board' - It's not too expensive and you can use it over and over and over. Lay your paper out on the board and STAPLE (yes) the paper down. I would use a light duty staple gun made for crafting, but not an office stapler and a heavy duty one might damage more than help. Do this the same way you would stretch canvas or put on lug nuts - staple the top right, then the bottom left, top left, then bottom right. Go around like this, working out the water as you go until the whole thing is stretched. Yes, you should really be stretching this stuff. Be careful not to rip it, but the idea is to stretch the soaking wet fibers out as far as they'll go, then let it dry in the shape so when you re-wet it, the fibers won't freak out get all crazy on you - they'll already know what to do
There's variations of this technique that involve swapping out some materials. You can use one of those particle -board drawing boards (I have one with a handle cut into it and a set of metal clips at the top to hold a pad of paper) and something called 'gummed tape'. The gummed tape looks like brown package tape, but it's activated by the water in the paper (or any water) so it doesn't stick until you tell it to.
2) Heavier paper. This is pretty simple. Watercolor papers come in all sorts of styles and textures. I find that working with heavy paper is the only way to go. The most popular is #140. It's a good weight, but it's still within most people's price-point. #90 should ONLY be used for quick studies an student work - it's not that it's a bad paper, it's just SO thin. #300 is my personal favorite, though some brands - Like Arches - make even heavier papers up to #555!
3) They make some items that hold down the edges of your paper for you while you paint by clamping them between a border and a supporting board and adjusted with screws. They only come in limited sizes, but I hear they work pretty well. Here's some, http://www.jerrysartarama.com/art-supply-stores/online/6894
Lastly, you could try pinning down the edges of your paper with the gummed tape (not soaking it, but make sure you let the tape dry completely before proceeding) and then coating the paper with a layer of acrylic medium. This can be almost any type of medium - acrylic gesso, matte medium, whatever you'd like. This will make an isolation layer between the wet acrylics and the paper. Keep it taped down and let it completely dry - even longer than you would normally let something completely dry. You probably won't get the same affect of the paint-on-paper, but it will help keep it from buckling.
As for masking the edges for a border, there's certain types of tapes that are called 'watercolor wash out' tapes. You could also probably use the gummed tape for this. Tape off the edges, do your painting and then peel off while it's still wet. Unlike watercolors, acrylics form a tough paint film that will be tattered and torn if you wait until the paint is dry to peel it.
I hope this helps!