And then sort out your supplies.
Line up the two pieces, (one short and one long) And make sure the short piece has two holes on the bottom.
Find a screw bit that matches the long screws. Then screw them into the two pieces that you matched up earlier. x2
Put a wooden peg into the two outside holes of the smallest rectangle piece.
Put 4 wooden pegs into the 4 holes on the longest rectangle piece (the thin one) And push it into the shortest side piece.
Put two wooden pegs through the holes in the middle of the thin, long, rectangle piece and push the second small piece onto those pegs.
Then attach the two side pieces using the wooden pegs. Like you did earlier, but do the steps backwards.
Then screw the pieces together with two a screw on each corner. And VOILA! You made a shelf.
I'll make another guide for how to hang the shelf on your wall!