There are many videos on YouTube that cover this topic, I haven't yet made a video on how I did it, but I did most of a step-by-step in my blog thread about how I made Bandit's cage. It starts about 1/4 of the way down this page here and is over several pages:
http://www.rabbitsonline.net/showthread.php?t=73547&page=39
The base of the cage can be variable, some people (like myself) use plywood or braceboard and cover it with a waterproof surface such as cut to measure vinyl flooring. Other people don't put a base and simply have fleece blankets that are big enough to cover the bottom of the cage and outside the edges so the cage sits on the fleece. This is a much more cost effective option, although it has limitations if you have a bunny that constantly pulls up the blanket or has bad litter habits.
For one bunny (of medium size) I would suggest a minimum of four grids long, two deep, and three tall. Mine is similar, just five grids long. My bottom level is two grids high which gives plenty of room for stretching whereas the top level is only one grid high (still high enough for hopping around depending on your rabbit). Some people split it evenly so that each level is 1.5 grids high. To add a second level, you can either attach a layer of the grids and rest your flooring on it (may need a beam for support), or you can simply support flooring (plywood or similar) with wooden dowels (bunny safe wood of course).
If you're getting two bunnies, I would suggest at least 5 grids long and 4 grids high (Unless you're getting something like netherland dwarf, but still, if you have the space, the suggested size is to get the most expensive cage you can afford that fits in the space available. Also known as, bigger is better!
So they can be a bit difficult to vacuum.
