Glue two sliders into each rail as shown in the diagram. One slider goes below the peg slot, and the other is on the opposite side, opposite end.
Remember that half of your rails are mirror images of the other half.
If you made half of the sliders slightly longer as described above, put the longer slider at the peg slot end.
One slider goes at the back of each front rail, and at the front of each back rail. Again, you're making mirror images here. It might be a good idea to hold off on gluing the sliders into the front and rear rails until you're certain you've got the right ends.