I use a sieve on top of a large baking tray. Shake it around a bit then pick out the small mealworms that got through the holes.
I would not recommend doing this outdoors as you will lose small mealworms depending on the size of holes in your sieve.
Shaking the tray (as guided by Hive) does not work as the holes are way too small.
Using the harvest function (as guided by Hive) is very inefficient.