Mark both ends of maxdrain.
Use hole saw to cut out room to work snips, leave 1" to turn down into maxdrain. The four corners are snipped up to 1/8 from pencil mark. (Allowance for thickness of maxdrain) Snip also should stop at edge of gutter for maximum opening.
Bend front 1" 90 degree angle
Score back with razor knife and bend back and forth once or twice to break off backside.(1" not needed)
Bend the 1" lap down 45 degree at each end.
Attach with pop rivets
Only need caulk at 4 corners
Tip: Bend Tabs out to hold next connection and to max the opening.