This is very easy to do. Some of these steps may be common sense, but I'll include them all for anyone without any experience at all!
Growing a square pumpkin is fun and a great way to get kids interested in gardening.
(Note: The process is the same for growing watermelon and gourds into a cube or "square shape" as well.)
Step 1: Acquire Pumpkin Seeds
With dozens of pumpkin varieties out there, you've got a lot of options so long as the pumpkin is large enough to fill the container.
Tom Fox, Racer, Racer Plus PMR, Rival PMR, Charisma PMR, Howden, Rock Star, Expert, Champion, or any "Jack O' Latern" pumpkin variety will work fine.
Step 2: Plant Your Seeds
Step 3: Putting the Pumpkin in the Box
If you have a few pumpkins to choose from, go with one that is still fully green but just fits inside the box. By, choosing a pumpkin that is already beginning to ripen, you risk the pumpkin not growing large enough to fill out the box and take on a square shape.
A fully green pumpkin that just barely fits in the box, but still allows the box to be closed will grow into the box fully.
Once inside the box, secure and fasten all holes on the box.
THIS IS IMPORTANT!
If you do not secure all the holes, the pumpkin will expand and apply pressure disproportionately causing the box to break. (See heart shaped pumpkin pictures)
Holes can be fastened with the nuts and bolts provided in the molds, or you can use zip ties from Lowe's or Home Depot.
Step 4: Monitor Your Pumpkin
Once you've selected your pumpkin and have it fastened in your box, keep an eye on it.
The box has drainage holes for excess moisture, but for people in climates with lots of rain, its important that the pumpkin doesn't sit in water trapped inside the box.
Condensation forming during the night and into the morning is normal and will drain out or evaporate on its own.