It looks like you're going to cast 16 stitches onto the end of one of your needles.
Then you'll work across those 16, plus 5 from the ones on the needle originally. You'll turn the work then and purl back, working the first 5 from the other needle.
On the next row, you'll work the stitches you just did plus 5 more.
Each time you turn, you'll knit 5 more stitches from the other needle that haven't been knit yet.
On the final rows, you'll include the final 6 from each side and will now have all your stitches on the needle.
[COLOR=Red]AT THE SAME TIME [/COLOR]you're doing this, you'll be decreasing at each edge of the gusset (the center 16) so that you'll use them up and end up with 52 stitches again.
On the next row you'll work across the 21 you just knit plus 5 more.
Keep doing this until you've knit two more knit rows where you've picked up 5 each time.
Then you'll do it again and knit all the stitches.