The simplest system would be a rectangular basement. You run two 10 or 12" plenums parrallel to the footer in the long direction, one on each side of the floor. Turn up you supply riser on the end of one plenum, and your return on the opposite end of the other. That way the flow is balanced through the system. Then you run 3 or 4" diameter tubes from plenum to plenum on 12 to 16" centers. Drill holes in the plenums with a hole saw and use standard flex pipe couplings to snap into the holes. I like to foam around the fittings to seal them.

I try to lay out the system so the pipes and plenums are flush on top, then backfill with sand to wthin 2" of the tops of the tubes. Next you roll out #10 wire fabric over top of the tubes. Tie your PEX directly to the wire fabric. When you pour your floor, the flex pipe ends up embedded about half way in the slab.

The amount of insulation you put under the slab is a little controversial. I recommend 2" EPX witnin 48" of the edges, and 1" everywhere else. This allows the floor to act as a cooling unit in the summer, without allowing significant heat loss in the winter. Don't forget to insulate the tops of the footers and edge of the slab.