Then on the opposite side (lengthwise) pull the fabric tight across the face of the panel in the center, and put a few staples in to hold the fabric tight.
Although there aren't more photos of the rest. Starting at the middle next to the staples you just made, pull small sections tight across the face, while at the same time slightly pulling it away from the staples you made, to make it tight in both directions. Do this in sections of 6-8" or so, and run a line of 5-6 staples. Then continue doing this toward the corner until you reach it.
Once you reach the corner, go back to the center and work the other direction.
Once both sides (lengthwise) are stapled, go to the top, wrap the fabric around the top edge, and staple a few times in the center.
Then go the opposite side (the bottom) and pull the fabric tight across the face, and put another three staples in the bottom center. Now work from the center staples on both the top and the bottom towards the corners like your did the sides. Make sure that you don't staple too close to the corner itself, as you will need to fold over and staple the remaining fabric like wrapping paper corners.
Cut off the excess fabric (from the staples) and you have a completed panel!