For a permanent ledge:
Get a bunch of cinder blocks, a bunch of quick-crete, water for the quick-crete, some ply wood, and some angle iron the length of the ledge. Cinder blocks are 8" high, 8" wide, and 16" long, so you have to use multiples of that. A good height for a manny pad is 8" high (so only one cinder block high), but use two cinder blocks high (16") for a nice grindable ledge. I would say 2-3 cinder blocks wide for a ledge, since you still want the option to do manuals on it. The length is all up to you. 8 inches is pretty standard for grind combos, so that's about 7 cinder blocks long. Now, find the spot you want to make it, and lay down a layer of quick crete. Once it's all down (make sure you keep it wet so it doesn't dry out on you), lay the first layer of cinder blocks down, putting quick-crete between each of the blocks. Then, spread more quick-crete on top of those cinder blocks, and continue to lay the second layer of cinder blocks on top. Now add one more layer of quick-crete on top of those cinder blocks, and lay the plywood on top (this will make sure your wheels don't snag inside the holes of the cinder blocks) and then do the same for the angle iron. Wax and shred.
For a wooden (semi-portable ledge):
http://diyskate.com/ledge_01.html