I have been making my own juggling bags since high school, where I learned to juggle from the Klutz Press book (highly recommended) because I was in the play "Carnival." I have always made my bags in the shape of cubes and I was surprised to find that there isn't an Instructable for making these bags. I think these bags are easier to make than the more spherical shapes and are nicer to juggle than their pillow-shaped counterparts. It takes me roughly half an hour to make one bag from a blank piece of cloth. You can speed up the cutting and measuring when you're making a bunch at once.
Step 1: Cut Your Parts
These bags have only three parts, even though they have six faces, because one piece of material wraps all the way around the cube. First you must decide how large you want your bags. For simplicity's sake, we will shoot for a two inch face and use one quarter inch seam allowances. This means that the wrap-around piece should be 4x2=8 inches long for the faces, plus 2x0.25 inches for the seam allowance, leading to a total length of 8.5 inches. For the width, we need 2 inches for the face, plus two seam allowances, leading to 2.5 inches. I mark my measurements with pencil and cut on the lines (photo 1). I often use denim from old jeans for this strip because it can handle a lot of abuse.
Once you cut the wrap-around strip out, you can use it to measure for the small squares. First cut a long piece using the width of your strip as a guide, then turn the strip 90 degrees to cut the long piece into squares. I usually use something colorful for the end squares to make the bag look nice (this set uses a material from an old pair of pajama pants) but you can also use more denim.