I cut a 6 inch strip of plastic canvas. I created a curve that made the entire piece 1 - 1.25 inches wide.
Next I wrapped it in duct-tape. This helps prevent snags while inserting, makes it harder to see through the stitches and also provides a bit more stability so you can bend it.
Fold the knit fabric so the Cast on edge (CO) is on top. Insert your needle through the first stitch on the CO round and the first stitch on the round on the needles.
And pull through both stitches.
Eventually you will add the brim. the wrapped plastic canvas is bendable, use this to your advantage when trying to work around the stitches. Don't get too frustrated if it's tight and difficult. it will get easier and you can push the brim through more of the hat if needed. Push it out of your way, into the closed casing.