This is my first Instructable and I plan on entering it in the "Play with Clay Challenge" contest that's going on right now so please feel free to leave comments (or vote) or tell me how I can improve my entry.
Step 1: The Materials
-a writing instrument such as a pen or pencil (I let the force be with me and chose my Yoda minifigure pen)
-a cutting instrument such as a craft knife
-an adhesive of some sort such as glue or tape
-a few index cards or other strong, yet sturdy paper such as cardstock
-a ruler or straightedge
-a glob of clay in your choice of color
-a small bowl with water
-clay working tools of your choice such as cotton swabs, toothpicks, cutters, plastic wrap, rolling pin, etc.
Step 2: The Template
Your goal is to have something you can work from that will give you an open box shape. I also traced a piece to make the lid (the same size as the base) because I wanted it to fit really close to the rest of the box. The dashed line on the "lid" template is just to give a rough idea about where the lid support piece will go later on.
Step 3: Cut It Out!
Lightly, and I mean lightly, score the lines that you didn't cut along. This will help to make the edges of the box extra crisp which is important because adding clay will round the edges a bit.
Step 4: Time to Play With Clay
Once you have a thin sheet the size of the template, you can start to cover the box. I folded the clay over one of the short sides (labeled "end" on my template), making sure it stayed tight along the bottom edge and the top edge where it folded over to the inside of the box.
After securing the two short sides, the longer sides (labeled "side" on my template) need to be attached. I cut away some of the excess clay that was between the long side and the short side so that there wouldn't be any bulging.
I didn't have enough clay to line the inside of my box but if you do you should make another sheet to smoosh into the inside and smooth into the side pieces that overlap the top edge. Once all the sides are wrapped over the edge of the inside of the box, you can work on smoothing and points where the clay sides meet or other blemishes. Using a bit of water on your fingers helps to smooth without leaving fingerprints.
Step 5: The Lid
Decide which side is the ugly side and designate that as the bottom of the lid. Roll out a long, thin "worm" piece of clay that is long enough to line the lid near the area that had dash marks on the template. Attach the worm piece to the ugly side of the lid, making sure it is close to the edge of the lid but that when put onto the box part there is enough clearance to fit still.
Step 6: The Studs
Use another glob of clay to roll out a thicker "worm" than the one you used in the previous step. Make sure that the circumference of your "worm" is the same size that you want your studs to be.
Freeze the "worm" for a few minutes (this will make cutting it go a lot better). While you wait for the freezing process, it would be a good time to set your oven to whatever temperature the manufacturer of your clay suggests. Now use a craft knife to slice 8 studs. You could also try using a hard-boiled egg/banana/strawberry cutter to get more even slices.
Arrange the 8 studs on the top of the lid (opposite your ugly side with the skinny "worm") in a 2 by 4 pattern.
Step 7: The Final Countdown
TADA! You've now made your very own Lego Polymer Clay Box! Fill your box with treasures and admire your work.