Hi, welcome to GoY, seed is not the usual method of propagating this plant, are you sure they are Bougainvillea?, the usual method is to take softwood cuttings in early spring, or semi ripe cuttings in summer, in a propagator with some bottom heat, if you do have seedlings, the next step is to prick them out into individual pots, using john innes number 1, pot them on later into j i number 2, and finally into j i number 3, they're not hardy in the uk, so will have to be kept, preferably in a greenhouse, heated to a minimum of 45f, better still to 50-55, Derek.

wait until each seedling has 2 or 4 proper leaves. the first 2 leaves are seed leaves and if you prick them out with just these they don't usually survive.
welcome for me too :o)

Wow! Where did you get Bougainvillea seeds? Modern cultivated kinds are sterile hybrids, or bud sports of the same. Seeds would have to be collected from wild plants in South America.