First of all take your upholstery webbing. Fold the beginning of it so that it can't break and, using the staple gun, attach it to about one half of the wood frame, as you can see in the pictures.
The webbing must arrive to the other side of the frame so cut it where the wood ends. Pull the webbing as tight as you can and secure it with a few staples leaving a couple of cm (0.78 inches) free. Like you did at the beginning, fold the end and attach it. The only difference is that it will be upside down but don't worry.
Do the same on the other half of the frame.
Now rotate the frame and repeat the same process so that there will be 2 new strips perpendicular to the first ones. You can twist them like I did (see the pictures) but it's not really important.