Hi Tina :)

Place boric acid in a squeeze bottle, and remove the wall plates covering the electrical outlets. Sqqueeze the boric acid in the bottom of the outlets.
The cockroaches move between the walls and travel fron one apartment to another going in and out of the outlets in the dark. And under the doors in the hallway.

They track the boric acid to the nests. Cockroaches social critters and always groom each other, and ingest the boric acid. cockroaches cant throw up, thats why the boric acid kills them.

If your uses insecticide, it goes between the walls where electrical outlets are, and under doors at entranceway.