Now that the edge triangle has been worked, it's time to start working the purl side squares (again, just called purl squares because that's the side where most of the action is happening).
Now's the time all those slipped stitches on the base triangles will start to come in handy. Using the right-hand needle and with the purl side of the work facing you, pick up and purl 8 stitches along the side of the base triangle. You can work right into those slipped stitches so it should be really simple.
Of course if you're working with triangle bases larger than 8 stitches, pick up however many stitches are equal to the number of stitches you started each triangle with on the previous row.