Take your glue stick and glue each slide mount to the centre back of your pre-cut paper
Now fold each corner of the paper inward to the back so the point matched the inner centre corner (see pic) open the fold back up and cut along the fold line you just created. Fold each side back in and glue down
When they are all covered - go back to the first one and cut corner to corner on the diagonal across the inside.
Snip the top edge of each triangle, (to ensure that the paper you are folding back isnt wider than the slide) fold back and glue down. Do the same with all your slides.
Cover your ATC using same method.Cut from one corner to the other across the centre of each slide mount. Snip the top one third of each point off so that when you fold and glue them down - they aren't wider than the slide mount edges.
Take the glue stick and glue down. Continue until all your slide mounts are covered.