The next phase is to introduce the female betta fish to the male. I always do a indirect introduction first by placing the female in a jar and then put the jar in the breeding tank. The jar is normally only half filled having high walls that prevent either fish from jumping in or out, and at the same time, the female has no problems reaching the surface for air. This indirect introduction may take place concurrently with the male Betta fish building the nest. Depending on the fishes, the sight of a female can either spurs the male into building a nest faster, or simply just distract him from his job, so the breeder has to react accordingly.
When the female betta fish shows that she subverts to the male, by showing very distinct vertical stripes/bars and at times a drooping head, it is time to physically introduce her to the breeding tank. It does not matter how dark of colourful the female is before mating session, her strips will be very visible. There may be identification problems when it comes to light bodied fishes though.