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.
Step 2: Sew and Mark the Strip
Start your sewing by folding the long strip in half and stitching the ends together. Note that we are sewing these inside-out, so put the side you want to see on the outside of your bag on the inside of the fold. I measured assuming a 0.25" seam allowance, so I put the stitch 0.25" from the edge. I also usually zigzag over the ends to keep them from unraveling (although this may not be an issue on the inside of a bean bag).
Mark where your crease is from the original fold. Rearrange the fold by rolling the tube until your marks are over the seam and re-crease. Mark at the creases. You should now have your strip divided into four equal lengths between the marks (counting the seam as a mark).
Step 3: Sew on the Ends
Put one of the edge squares inside the tube and center it between two of your marks (photo 1). I usually start opposite the seam because it's a little easier to get it lined up this way, but it really doesn't matter. Note again that we are sewing the bag inside out, so place the face you want to see when it's done against the denim.
Sew from one mark to the other, reversing stitching at both ends to lock it in (photo 2).
Line up the next edge of the square with the denim and sew again from mark to mark (photo 3). Sometimes it seems easier to sew from the colored side, rather than the denim side, because you can pull the colored square taut and it's easier to see where to start. The down side is that you can't easily see where to stop. Try both ways and see which works better for you. I found myself sewing from the colored side when I made bags 2 and 3 for this Instructable.
When you have sewn all four edges, you can zigzag all the way around if you like.
Repeat with the other square at the other end, but only sew three of the four edges so you can turn the bag right side out and fill it.
Step 4: Fill and Close It Up
Fill the bag with your filling of choice. I usually use rice, but you can use beans, plastic beads, or whatever you like. I even have a set with slightly larger bags filled with steel shot, which makes them very heavy. These are good for a juggling workout and for really smoothing out your scoops (because otherwise catching them is very jarring).
Don't over stuff the bags - they feel best with a little bit of "give" to them.
Fold in the edges and hand-sew it shut. I usually lock the stitches together (photo 2) and tie it off every now and then (about every quarter of the way across the seam) so the whole thing doesn't pop open if the thread breaks (again, these things take a beating, especially when you have small children who like to throw them at each other).
Repeat these steps to make however many bags you can handle and enjoy your new juggling bags!