First of all, make sure you’re planting it in a sunny, warm location. Bougainvillea needs sun & heat to thrive & be a blooming machine.
This plant also loves the heat. Not enough sun = not enough color.
Just like planting a shrub, dig the hole at least 2 times as wide as the rootball. Loosen the soil on the bottom of the hole to make sure the water will drain out.
Bougainvillea doesn’t like to be kept sopping wet & is subject to root rot.
The soil needs to be well drained. A rich, loamy soil is ideal. I always amend in the ratio of 1/3 local organic compost to 2/3 native soil when planting bougainvillea.
I always keep a 2-3in layer of compost on top for good measure.
Bougainvillea prefers infrequent, deep waterings rather than frequent shallow waterings. Too much water = too much green growth & eventually rot.