No problem, Alison Joy! Let me see if I can remember...
I got a 2' x 4' piece of basic pine plywood at Lowe's for about $9 -- I cut that into a 20" by 20" section, then halved it diagonally to make the two shelves. (You could make it bigger or smaller, of course, but I was limited by my window frame, which was just over 20" away from that corner, lol.)
The sisal rope, which came in a 100' roll, was about $10 at Ace Hardware, and was plenty long enough to cover all three posts...I still have some left over, in fact.
The dowel/post thingie was an 8' long 2" x 2" dimensional lumber piece at Lowe's that only cost $3. I cut each piece into about 14 1/2" sections. But again, the height is up to you
A package or two of screws ran about $2 apiece -- I forget what size I used...I think it may have been 1 1/2 or 1/34" length and size 6 or 8 gauge...
And again, the carpet remnant was just $5. It actually came in a roll of 2" by 12", so I have pleeeenty left over. Thinking about throwing some down as a throw rug somewhere...
In fact, with the exception of the sisal, I have enough materials left over to duplicate this shelf again and make a whole new one! So, all told...just around or just over $30 spent on the project.
Oh, and other essential items that you should have on hand (not worth going out to buy just for a project like this!) would be a circular saw, a good utility knife (or two, as that carpet resents being cut!), a drill with a very small bit, a good power driver, and a staple gun that actually works, lol! (The staples didn't last long in that top back corner -- see first picture; I had to take the shelf down and tack down the carpet again with a plethora of tiny nails that I had on hand for hanging lightweight picture frames. Poor Imogen didn't know what to think of that
Also, the one difficult part of this was figuring out how to get the carpet edges folded over and attached on the sides of the very thin plywood (i.e., how to make it look seamless). I thought at first I would try sewing the two overlapping edges together, but I couldn't find a thread strong enough, so I ended up folding it very carefully and hammering a ton of those tiny nails very close together to hold it in place. If you take the shelf away from the wall you can see it looks a bit rough in places, but hey, Imogen doesn't mind!