My senior skit was power rangers, so this was our float. It's about 20ft tall.
First you will want to make blue prints.
We wanted to have the Megazord go from a sitting position to a standing position so I had to factor that in when making the sketches. I highly recommend using graph paper for this, to get all the measurements accurate. We decided to have a PVC pipe frame to support the cardboard.
I think PVC costs around $4 for 20ft pieces at 1 1/2 - 2" thickness. You would have to check your local hardware store. You also have to factor in all of the connecting pieces of pipe for the corners.
Second, I made a prototype, but unless you are fairly good at small crafts, this might get a little annoying to make. I just used posterboard and tape, using the blueprints from before as a guide.
This will help you visualize how your giant Megazord will move and such. We had to pull ropes over support bars at the top of our gym to make it stand up so it was nice to be able to visualize this.
Third, you will want to get supplies.
Now is the fun part. Go around to absolutely every store you can and ask for cardboard, the bigger boxes the better. Some places are nicer than others so don't get discouraged.
You will also need spray paint in whatever colors you choose to use, PVC, rope (if you are planning to hang it from somewhere), and glue and duct tape, especially PVC glue because the weight of the cardboard and PVC will try to pull apart the PVC.
Finally you get to make it.
Grab a few friends.
Cut your PVC to size, put it together, glue it in place.
Then take your cardboard and duct tape and glue it on the PVC.
Spray paint the whole thing.
Power Rangers Assemble!