(This is for a queen size bed. King-sizers, you're on your own with this one. Sorry.)
You will need:
Two hollow core doors (30" wide each)
Three 1" thick boards, 5" wide, cut to 50" long
Three 6" long steel connectors (flat pieces of steel with screw holes)
Washers and screws
Six hairpin legs and three metal brackets
One 1 1/2" x 18" wide board cut to 60" long
So. Lay your doors side by side. See? This is your bed platform. Now take your steel connectors and position one across the middle, and one on each end (at the head and foot of the bed). Set these two in about 10" from the head and foot. secure the doors together to make one 60" wide platform. Then take the 1 x 5s and lay them across the bed over the steel connectors. Now, using plenty of washers and screws, secure the boards to the doors. Make it burly. This is what will keep you from ending up on the floor after, um, excessive movement in your bed.
(Note: if you end up on the floor anyway, don't say I didn't warn you.)
Okay, now that your platform is done, rest the head on a chair or something so you can attach the brackets. This is what holds up the headboard. From the side with the boards on it, screw one bracket into the middle, and one on each side, set in about 8" from the edge of the platform. Take your 60" board and mark where the screw holes will be on the bracket, and drill some guide holes. We'll come back to this in a minute.
Now it's time to attach the legs. Square them with the corners of the boards on the platform. You have 6, so two will go on each board, one at either end. Screw 'em in. Use washers. Now stand the bed up. Are your headboard brackets sticking up, ready to receive the bounty of your pillows? If not, unscrew them and put them on the right way. There you go.
Go ahead and attach the headboard itself. If you took my advice and drilled guide holes, this will be easy. If not, you will have to spend some time in purgatory or find a friend to hold the headboard for you while you mark the screw holes and do it now. Go it all screwed on tight? Great!
Take a step back and really look at the bed. Do you think it will hold your weight? If you have doubts you may want to give it a test run before you start practicing back flips on it. I know a couple people who use doors for platform beds and it seems to work pretty well, but be careful all the same.
You can paint it or stain it, or leave it natural. And you can go to sleep with the lovely, warm feeling that you just saved yourself over a grand. Pleasant dreams!