Headliner adhesive:
Smoothed out with roller:
It is important to only apply adhesive up to about 1" from the edges. This allows for fine-tuning the fit under the trim. If adhesive is applied all the way to the edge, you'll never tuck the material under the trim.
Finished product, after tucking edges under the trim:
It's not "perfect", but given that it's not something that is really viewed with scrutiny by driver or passenger, it'll do for now. I want to see how my application technique holds up over time in the florida heat, before I decide to amend some of my errors (a few areas were trimmed too short, and there's a bubble here and there).