This ridge isn't exactly "picking up stitches"... I mean, it is, but not really what Amy describes. This is a special technique for this pattern to create that lip around the bottom of the jar.
Ok, you should be looking at the wrong side of the knitted fabric... the side with the purl bumps on it.
Look at the first stitch on your left needle, then count the purl bumps going down from there. Pick up the 6th bump by inserting your needle into it from bottom to top. (The pattern says to use your left needle, but it will be easier to pick it up with your right and move it to the left.)
Pull that stitch up and put in on your left needle. Then purl the original first stitch on your needle and that new picked up stitch together as one.
Hope this helps. If not, I'll try to go into more detail!