Hi Keith, here is a pic of how i did it.
With the crankcases and barrel lying side by side, and positioned to be exactly in line with each other it was straight forward to move the pistons up in to the barrel one at a time by rotating the crank. I used a ring compressor, which helped. It wasn't quick, it took a lot of care and and time to move each piston in slowly making sure it wasn't pinching and that the ring gaps were not aligned. Also it was hard to see what was happening on the side that was out of view.
Plan B was to put the pistons in the barrels before putting them on the con rods - as suggested above. Plan C was to get two helpers, one to hold the barrel and one to hold a crank.
Whichever way you end up doing it, take it slow and easy, have all the tools to hand, keep the work bench clean and clear, and set aside an hour or two free without interruptions. Good luck!
Ariel Square 4 mk2, Ariel Colt, Matchless G12, BSA C12. Room for one more......