It's quite a simple build, just get a bed and put it on some kind of wooden stilts ( this can be done many ways) and add some wooden boards on the sides to create what I like to call "The Den" (see images if I'm not making sense) if using a double bed you can half the space under and use half of it for storage and the other half for an bed/ gaming area e.c.t.
Step 2: Furnishings
Once the main body is complete, (run some speakers inside and setup an xbox as well like I did for an awesome gaming experience) I used some black weed barrier fabric on the inside to make it look really cool and some LED strip lighting (£15 from eBay) with a couple bean bags and it starts to look really cool. You can do this build with a double or single bed, I know because I did this as a single first but my friends and I used it a lot so I turned it into a double so we could all get in, also a few of my friends have slept in there when staying round and all said how clever and cool it was. Please have a go at this build you won't regret it, it's practical and pretty darn awesome