There are various ways you could rig this
The setup here shows a 2:1 purchase between boom and traveler (triangle).
The block at the apex of the triangle is knotted to the traveler.
There are several ways to adjust the traveler. A cheap one is to tie the two ends together so that the traveler is a fixed triangle. Make sure the knot has a loop - that loop can be hooked over a hook, perhaps under the coaming, to "center" the traveler. When released, the knot can travel between the two blocks on the transom, allowing the apex to swing off-center for downwind runs (requiring less mainsheet).
You could run both tails through a dual sheet block on the deck, and lead them forward. Then knot or splice them together about a foot from the cheek block and provide some cleat for the remaining single tail. That way, you can adjust the height of the triangle, while keeping it always centered.
There are many other interesting sheeting arrangements that you can use those transom blocks for. For example, you can simply run the main to a block at the end of the boom, then through both blocks on the transom, then back up to the end of the boom (and attach there). This gives you a similar arrangement, but without a way to arrest or adjust the traveler. That's how my boat used to be set up when I got it.
Mine is now rigged like in the top picture. I was following the rigging guide by Phill Root
http://forum.daysailer.org/tech_rigguide.phpwho also supplied the bottom pictures, but they seem to show the variant with cheek block on deck, not the fixed triangle traveler with hook under the coaming shown in his guide.