I knitted the 20 rows on my circular needles. Now how do I pick up the side and bottom stitches??
The section that you just knit is the bottom of the bag.
Hold the piece so your live stitches are at the bottom and you're going to pick up the stitches up the side where your yarn is attached, across the top, and down the other side.
There's a video under the Free Videos tab at the top of the page, Tips, Picking up stitches that may help too.
Turn the square to the right (not to the other side, sideways) so the sts on your needle are on the right edge, then pick up across what's now the top edge, across the cast on edge and up the other side to where the other needle is. Then place a marker and knit around and around.