Not quite sure how leaders work. Could you please explain?
I've sewn some fabric scrap around the quilt when it was not
big enough to snap. Is that it?

Similar idea, yes.

Leaders are pieces of fabric longarmers use to attach quilts to longarm frames. Usually they have velcro sewn on the sides that attach to the rollers on the frame, with the corresponding velcro glued onto the rollers. You pull the leaders off the roller bars, attach the quilt to the leaders, then reattach the leaders to the rollers. There are several methods of attaching the quilt to the leaders; some people pin, some people use long basting stitches, some people sew on zippers.

I think there are other ways of attaching the leaders to the rollers, but most of my quilting friends are in the velcro camp, so that's the extent of my knowledge base. :)

Peckish, thanks for the explanation. I'll have to give it some thought on how
to make it work with the Q-snap.

If the quilts are that small (less than table-top size) - wouldn't it be easier to just put the layers on a table and then baste them?

I have a hand held snap-q. I made sleeves to slide on one or two sides for when I have to quilt the edges or corners of the quilt. I safety pin the edge of the quilt to these sleeves to keep the tension. But my snaps don't have any ridges on the inside, they are completely smooth. Ladies used to pin the edges of the quilt to the big old quilt frames in basically the same manner. The one old 1 that I saw the ladies put batting on the rails and covered it with fabric to pin to.