Right, finally getting my act together and writing this tutorial for using/attaching a bag strap slider. It is easy, easier than it might look anyway! One thing worth mentioning before we begin: make sure your strap will actually fit in the slider. This may be self-evident, but there we go, I said it anyway. Oh and, in case you're wondering, I got my slider from U-Handbag.
Take one end of your strap and pull it through the slider. Then stitch the end onto the strap. Back of the strap end to back of the strap. In this picture the front of the strap is the blue side and the orange is the back. The strap end after the slider can be as long or short as you like, but I find that if you're using a sewing machine to stitch it, you need room to move.
Now pull the strap though the loop on the bag (I used a D ring but you could have a round one or a leather loop or whatever), with the back of the strap (orange side here) facing you and the front (blue) facing the bag.
Pull the unattached end of the strap through the slider with front side facing you.
Finally, pull the unattached end through your other loop and attach like you did with the first one. And that's it. You're done!