I bought one yard of fleece fabric, 2 large buttons, elastic strapping, and webbing. I used thread I already had, and my sewing machine, that can do a zig-zag and straight stitch (among others, but those are the ones you need for this project). This cost less than $5, because it is spring and the fleece was on sale, besides the fact that I had a great coupon!
Start by making sure your fleece is square. Most of the time when they cut fabric they manage to make it uneven, so try to start with a square piece as much as possible. This will help your rug to lay right.
Cut a piece of "regular" fabric - a cotton quilter's type fabric that doesn't stretch, about 2" by 24", and fold it in half twice, so it is now .5x24" Line this fabric up at the middle of the cut edge of your fleece, and fold the fleece over twice to make a 1/2" seam covering the cotton fabric. This will keep the rump rug from stretching out of shape where you attach it to the saddle.
Now Measure your saddle from the back billet strap to where you want your rump rug to start. For me, this was 12.5" - I made a loop for the billet strap to go through, but now I greatly wish I had used velcro, instead! I may rip out the stitch and sew on the velcro yet....we'll see how ambitious I get. This is where you need the zig-zag stitch, to keep the end of the webbing from fraying. You can see that I did a double stitch, and went back and forth several times to make sure the seam was plenty strong. I added my webbing straps 10" from the center, which made the straps 20" apart. I could have put them closer together, but this will work fine.
After you've sewn on the webbing to attach the rug to the billets, add the buttons to the top of the rug, just inside of where you stitched on the webbing. Just use a needle and thread and sew them on like any other button. Now cut your elastic to the length you desire (mine was 15"), and stitch that onto the rug just inside of the buttons on either side. In this picture I'm holding up the elastic on the off-side of the rug. It's short enough it doesn't get in the way, long enough and elastic enough that it is easy to use to "tie up" the rug when I don't need it.
Here it is a bit further zoomed out: you can see the webbing, the button, and the elastic strap.
Here is the rump rug with the off side tied back. This is very simple to do one-handed and while you're in the saddle. No need to tie any bows or knots, just reach back and gather the rug, slip the elastic loop underneath, and loop it over the button, making sure it's secure. That's it!
So now I've tried the rug on Arabee for length and I have since finished the other three sides of the rump rug. I cut the selvage edge off (about a 1/2 inch on either side) and cut a quarter circle off the back to round the corners. Then I cut 1" x 4" strips all around the rug except for the front, and tied those strips in knots, to finish the fleece rug similar to a blanket. I don't have a "finished" picture of it on her yet, so just imagine it like this only with the knotted edge around the sides and back.
Now, this isn't the only way to make a rump rug, by any means, but it is how I made mine. I think the best part of the one I made is the buttons and elastic loop to tie the rug back when it's not needed. And also, the less than $5 price tag!! The fleece was priced at $3/yard, but it was 50% off of that even! I thought the apple print (even though they are funky colors) was fitting for Arabee, since like many horses she enjoys the occasional apple treat :-)
**Note: The stirrup covers you see on the saddle are ACTUALLY halter fleece tubing! The nose piece was 12" (perfect for me) and the crown was 4" longer, but I cut it to match. Not bad for an old tack box find repurposed!
Hope this little tutorial helps. If you make one of your own, I'd love to see photos - post them on your blogs and link to them in the comments section.