Here's a pic that I hope helps.
There's a few places online to buy the stuff; strapworks.com is one. You can find it at Wal-Mart, or Joann Fabrics, but it seems a little more costly.
As for measurements, I start with 72", cut it at 42 for the upper strap, 30 for the lower. Burn the ends of the nylon strap to keep it from fraying.
You're going to sew a female end of a Side Release Buckle onto the end of the 42" strap; give yourself 8 or so inches of overlap, and sew at least two box stitches ... looks like a box with an x in it ... one right next to the buckle, and the other one on the end of the overlap. This forms a section of the cuff and keeps it from being just a simple tourniquet, if you've ever seen someone lose circulation in their left arm from loop sling shooting.
Now sew a male end of an SRB onto the end of the 30" strap; you only need enough overlap for one box stitch, like 2" or so. This may be the adjustable side, in which case run the strap from under the end, around the center then back to the under side, so when you stitch it its on the hidden side of the strap - closest to the gun.
Clip the straps together and lay the whole thing out. The "ugly" side with the overlapped sections should be facing the ground, and the "pretty" side up.
Now disconnect the straps, and reverse the top strap end to end so the female end is on the opposite end. Now find the other female end and clip it to the lower strap.
Take the end of the upper strap, feed it through the top of the female end of the SRB on the lower strap, and pull it through until you get to the doubled over section ... roughly 6" from the other female end.
You should have one adjustable male end left; feed that onto the upper strap with the tip facing down, and slide it down until you can clip it into its own end.
Now you have two tri-glides left; one goes on each end, thread it from the underside of one end, over the center, then under the other end, loop it through a swivel end, then back through the tri-glide. This is how you adjust each strap for length. Depending on the length of the rifle, you will probably wind up trimming off the ends so you don't have a lot of extra hanging around.
It makes a lot of sense when you lay it all out, I promise!
The really PITB is figuring out the sewing machine; I wound up hand turning the action for the whole thing. I highly recommend you run a few stitches on a piece of strap so you can adjust the tension on the machine. Use a #16 or #18 machine needle, and heavy polyester thread. You do not need to double up the stitch, if you use the box stitch, just make sure you lock the ends of the stitch. Here's a tutorial on the stitch: https://www.youtube.com/watch?v=y03wcd_Z1G4