When making the bra strap, the tricky bit is making it so it lies flat and not twisted, get it right and your bra is finished, as usually it's the last component to sew, and the last thing you want to do is be unpicking mistakes at the end.
When choosing a bra strap size, the slider has to the correct same width as the bra strap (can be slightly bigger) but the ring can be slightly smaller if needed. The reason for the ring is so the strap can move with wear that's why the rings are usually placed at the back.
1. With your strap length cut, thread the slider on to the strap and keep the slider near the end.
2. Fold the end strap towards the main body of the strap (so the wrong sides of the strap are touching) and sew back and forth to secure.
3. Thread a ring onto the strap (other end of slider).
4. Holding the end of strap which the slider is attached thread the strap up through the slider (the two wrong sides should be facing one another so you hide the stitching end you did in step 2) over the strap and down through the other side of the slider.
And that's it.