How did the Enterprise return to their own timeline at the end of First Contact?

The Borg sphere's approach to time travel is described in two lines of the movie:

DATA: Sensors show chronometric particles emanating from the sphere.

PICARD: They're creating a temporal vortex.

The troublesome question of how the Enterprise returns is dealt with just as abruptly
at the end of the film:

LAFORGE: Captain. I've reconfigured our warp field to match the chronometric
readings of the Borg sphere.

PICARD: Re-create the vortex, commander.

So the Federation may not have the knowledge to make their own custom time vortex, but they can at least copy what the Borg have done. A case of "monkey see, monkey do".