Eddie:
You take a big piece of paper cut to fit around the barrel ( I used red "rosin" paper they sell at Lowes for under hardwood flooring) to lay out the pattern. I used 6 rows vertically, 8 slots in each row. The slots are about 8" long. You drill a 1/2-3/4" hole at the end of each place a slot is going, then use a jig or recip saw to make the 8" long slot between holes.
Then you start heating the plastic with a propane torch until it gets reasonably soft, then stick a foot or so hunk of 2x4 (miter the end off to a 45 makes it easier to get in the slot) in the slot, and bend it outward to form that lower "pocket", applying a little heat as needed to make it do what you want. Let it sit a few minutes to cool, and the pocket will stay formed. I used a 1/2 dozen 2x4 pcs so I could keep working ahead while the first ones cooled, then used the first ones.
There are several YouTube videos out there on Strawberry Barrels showing exactly how it's done.....I simply copied their idea on these.