I think it will depend upon the type of cage you have, but this is how I made mine in a rabbit 120...
I wanted to make the upper floor as big as possible (as the cage turned out to not be as big as it looked in the shop!) My upper floor is half as big as the cage.
The floor itself is made from mdf (I wasn't 100% sure what wood to use and was initially going to go for ply, but I figured the girls will never actually get to it as it's covered in fleece. MDF is strong and cheap so it won). Anyway, the floor is screwed onto two support pieces made from inch wide flat dowel. These supports sit on the bars of the cage and hold the floor in place. They stick out about 3/4 inch at either side, but this is ideal so there's no way the floor can fall down. I was going to use cable ties for added security, but I'm glad I didn't now as it's completley removable and makes cleaning out so much easier
I made a little 'fence' from some extra wood to stop any kamikaze jumps off the side.
I can't take credit for the ramp, it was 'borrowed' from their summer hutch. Since these photos were taken, I've made a couple of small changes, as they were taken pretty much as soon as it went in. I've added a little tunnel over the ramp made out of two lots of the bendy twig thingys. They've got vetbed on the lower 'landing' (the landing came with the cage) to make a softer landing pad. They initially had a little ramp up to here but they never used it and always jumped up so I just got rid of it
I'm pleased how it's turned out as they have almost half a cage extra floor space and the cage without just isn't big enough for three (well, two and a half
)
Looking down the cage to Pixel snoozing (only one twiggy tunnel here - Pixel has taken to climbing on them rolleyes
I hope that makes sense, I can get pics of the floor out of the cage if that's help - it's clean out time later tonight anyway