When the plant has popped out of the jiffy peat pellet, transfer it into a pot with fresh garden soil.
Fill up the pot with soil, make a deep hole in the middle and insert the whole jiffy peat pellet with the young plant.
Put it quite deep into that pot, bury half the stem for additional support. The buried stem will grow roots.
Gently and lightly compress the soil and slowly give a bit of water.
And put this back under your light.