Just got it myself
First, two candyes at log at righ from start, then two chocolates at middle (jump down from log at starting area) then back up, to left side to take everything from there (stunn patato at lover log, not bother with one on upper). Then jump down on left border of screen to colect few candyes in the air. And then move to the right side colecting rest of stuff.
Maybe need to do few runs to see what and how to collect, but you will get it . Good luck.

There's a Commander Keen wiki that has a map of each level in each game. That's really useful as you can plan your route to get all the items.