An old end table ( or u may need a full dinning room table if u are building it for a horse or giant dog!) i got mine for free outta the throw out pile at my sisters house. Usually you can find them for free or next to nothing at thrift stores or on craigslist.
Decorations ( i used an "L" I bought at Hobby Lobby for around two dollars)
Some kind of short legs or chunks of wood to lift the bed off the ground (i found some rounded little nubs at hobby lobby 4 for 3$ perfect!)
Hot glue comes in handy
and lastly an old pillow or something soft to use as the "Mattress"
This is really simple, you just flip the table over and take out the drawer then remove everything else that isn't needed ie: extra supports or screws and such. Then remove one end of the bed which will be the foot of the bed... This should also have a hole in it from the drawer, i took this piece and the bottom out the drawer and put it all together to make a larger headboard at the head of the bed. Then take your nubs or short legs and attach them to the bottom to get the bed off the ground. I used hot glue and screws. After that decorate however you like, paint, cloth, letters, or whatever! Then throw the pillow in it and you have one happy pup in a nearly free bed that looks awesome!