You may have landed here because you were reading my "Better, Faster, Easier Way To Make Suspenders" post... if not, definitely check it out. In that tutorial I am using the vest buckles commonly found at craft stores. This is just a quick visual of how you thread the suspender slides that look more professional. The slides that I use have prongs which might be different than other suspenders slides you will find -- I buy them from Crafty Munchkins on Etsy.
So here is what you will need. The two straps which you have already folded, pressed and topstitched. Four suspenders clips and the two suspender slides.
Here is a close up of the slides:
Take one of the straps and place it wrong side up on the flat part of the slide, and close the prongs. At first I wasn't convinced the prongs would hold but I haven't had any issues thus far!
Place the other end through the clip, going from back to front so the right side shows (as pictured).
Feed the loose end through the slide as pictured. You might have to swivel the clamp part up to create a path for the strap.
Now you have this (don't hurt your neck, sorry I didn't rotate the photo before deleting it from my phone!)
And stitch a zigzag or satin stitch to close it up.