Thin plywood sheets were screwed to the joists to create plain solid surface

Thin plywood sheets (3/16") were screwed up to the joists to create plain solid surface and the chalk line was snapped through the light fixture parallel to the long wall.

The first raw of Polystyrene ceiling tiles was installed along the chalk line

Row by row I worked my way to the sides

The joints between the tiles were filled with white paintable silicone.