The sites I relied on for info on laying out the PEX tubing were:
I can't recall the exact R-factor of the styro I put in the floor, but it was 5 inches of Hi-Density (closed cell) styro, which I purchased at a local supplier of hydronic heating components. I remember that I looked up what the code called for (which turned out to be an R-factor of "whatever", which translated to four inches for this climate zone) and went 25% better. (I have an annoying tendency to over-build everything I do)
Then a bunch more research into "which type and brand of PEX is best" for this use - so far as I can tell, they are ALL good, but some are even better, although price is not a guide. The most expensive brand is not well-regarded by local installers. One brand kept coming up as "best", and I was told that it is NOT the red or white stuff, but is orange, so I went with that one. Add in a couple of bags (1,000 in each bag) of zip ties so you can attach the PEX to the mesh and/or rebar and allow at least 3 full days of messing around to get the layout exact - after you pour the concrete there are no second chances!
Fill 'em with anti-freeze/water mix, and start installing the couplings on the ends. When you have all the "cold" ends plugged and sealed, hit the "wearm ends" with 175 psi air and wait 5 - 10 minutes for any possible leaks to appear. No leaks? Good! Release the pressure, cap the hot end of THAT loop, and move on to the next one. If you DO find a leak (I didn't) then replace that whole loop! You do NOT want to have joints inside your concrete slab!
(At least, I don't!)