These look deceptively simple, and like anything else, once you know how to do it, it seems like no big deal. However, the fact that there are a myriad of tutorials and videos about how to do it hints at the idea that it is tricker than it looks.
At least part of the problem is that most of the videos I have watched either use terms you have never heard of (fid, brummel), or assume that you have already watched some other video that you wonder where to find. Here are some resources that I found helpful:
The first job is to splice (that's right, splice) a 3 inch loop into one end. The foundation of this spice is what is called a "locked brummel" - I had never heard of it before either, but the videos make it clear how to create one, and actually doing it makes it even clearer. With the locked brummel section, it is clear that the strength of the loop does not just depend on the friction of the rope within itself. I had a 1 inch taper on the end, 4 inches buried, and 6 inches for the loop itself (mark at 1, 5, and 11).
The second link above shows an excellent method to avoid driving yourself crazy, namely to use a doubled over piece of wire as a fish tool. I cannot recommend this too much. I used a 24 inch piece of solid 22 gauge wire, and it was absolutely perfect. The guy who made the video used steel wire left over from a drop ceiling project.
After making the fixed loop, I measured from the end of the loop 12 inches and then 22 inches to allow a 10 inch friction section. I also added a plastic bead like I have seen on some of the whoopee slings people make. The unlooped end is finished by using the fish tool to pull a few inches inside of itself. This makes a nice finish and fattens up that end of the sling.
Tom's hiking pages / firstname.lastname@example.org