To begin, I advise not cutting the end of your weaving until after you have shoved it through the slider buckle. The amount of manipulating you have to do will cause the weaving to come undone a bit, and you'll just end up cutting it again anyway. The bulkier the fabric, the worse the problem, because it takes more effort to squeeze past and will unravel more easily.
I always start with the adjustable end. This makes the final max strap length measurement more accurate.
Note: If you have an asymmetrical pattern, you will need to pick which side of the pattern will be toward the shoulder. I assume most people wear a strap with the adjustable end toward the bottom of the guitar. I generally place the darker side so that it will lie to the outside (away from the persons neck) as I feel this gives a more balanced look. Decide how the strap will be worn, and which side should be toward the outside, and place the slider buckle accordingly.
1. Place the end of the strap through a hole in the slider buckle.
2. Place the end of the strap through the second hole in the slider buckle.
3. Place the end of the strap through the loop of a leather end. If there is a preferred front to the leather end, make sure the front is facing the correct way.
4. Pull an extra length of strap through the buckle to allow room to work.
5. Push the end of the strap back through the first hole of the buckle.
6. Push the end of the strap back down through the second hole of the buckle.
7. Cut off the unusable part of the cloth.
8. Fold the end of the strap down one time roughly 1/2 inch and sew in place.
Note: I have the end of the strap pulled out several inches. This is to allow room to sew this hem. The end of the strap will need to be pulled back toward the buckle for step 9.
9. Place the end of the strap flush with the strap that was looped through buckle hole one the second time and sew in place.
Sorry if that's a confusing way to say that, but I'm not sure how to state it any other way. Hopefully the picture makes it clear.
Here's a close up of the sewn side. I aim to pull the end of the strap through enough so that the folded over portion will be entirely beyond the buckle when all layers of cloth are pushed flat as shown above. I think this makes this area lay more flat when worn.
Here's picture of the other side of this area at the same place in sewing as the previous 2 pictures. I sew a line right near the buckle and another along the end of the strap. I also sew along the sides, sometimes wrapping the thread around the outside. If you look closely, you can see where I went around the outside in the last picture.
One last picture of this step from this strap.
Here's a photo of this step completed. This is a strap I did before I started pulling the end out further so the entire hem is very near the buckle.
Here's another view of the join on this older strap.
10. This is really multiple steps but it sums up to 'sew the other leather end on'
a. Slide the slider buckle as far as it will go against the free floating leather end.
b. Measure the cloth and cut to length. I think standard straps are 55 inches. Base guitar straps tend to be a bit longer. Be sure to add a few inches for sewing on the other leather end.
c. Slide the loop of the second leather end onto the free end of the cloth. Again, place the correct side upwards if there is a front face to the leather end.
d. Fold down the end of the cloth around 1/2 inch and sew into place. (Technically, you could wait to place the leather end on until after this step.)
e. Push the sewn edge against the cloth to the other side. Sew in place.
That's it! You should now have a brand new, unique, handmade guitar strap to use and/or share with your musician friends :)