Now that you have finished culling the dragonfruit seedlings and you have only the best in hand, you want to pot the individual dragonfruit seedlings in their own individual pots. For potting dragon fruit cactus seedlings, you can use the same soil mixture you used as the base soil mix for sprouting the seedlings.
Note: Use these same procedures for potting dragon fruit stem cuttings with a larger pot.
A 3-5 inch pot should be ample in size for this step (for re-potting dragon fruit vine cuttings, you will likely be using a larger pot as your cutting should be significantly larger than your dragon fruit seedlings at the potting stage). Making sure there is good drainage, fill the pot with the same soil medium you used to sprout the seedlings. Gently transplant into the pot one of the seedlings (keeping the handling of the dragonfruit seedlings root mass to a minimum).
Note: If you are potting cured dragonfruit stem cuttings, you will be using a larger pot as your dragonfruit cutting should be significantly larger than the dragonfruit seedlings you are transplanting here.
Initially, keep the repotted dragonfruit seedlings in approximately the same degree of sunlight as when they were in the tray. This will help to reduce the shock to the dragonfruit seedlings from being repotted. After a few days if not sooner, you can begin to slowly increase the amount of sunlight the repotted dragonfruit cactus seedlings get each day. Remember, as the dragonfruit cactus seedlings grow, they are getting more and more roots and will tolerate and want more sunlight and will also be able to better tolerate other stresses (periods of dryness, wind, etc.). Again, err on the side of keeping them a little dry to avoid rot. Do not keep the pots sodden with water! The dragonfruit cactus itself stores a lot of water, so as the dragonfruit plants begin to grow, they will have lots of water stored inside to handle any dry conditions - what they can't handle nearly so well is too much water. This holds true of course for all cacti and succulents...
After a few weeks or longer (allow at least several weeks of growing in the pots), you should be able to plant the dragonfruit cactus in its final location. As long as the repotted dragonfruit cactus is continuing to grow well and not getting pot-bound, you need not worry too much about planting it right away! If you are wanting to grow dragon fruit in a pot all the way to maturity, just keep repeating this procedure as the dragon fruit cactus outgrows its pot!