I take a sharp knife and cut around the stem and center vein and dispose of damaged areas and junk. Wash and cut good stuff into 1" wide strips.

Boil or steam. Three minutes may be enough if you like slightly crunchy. I steam 5 minutes, but I live at near 1 mile elevation. Sea level cooking is quicker.

The Kohlrabi stem is supposed to be the goal. Tastes great but is best harvested just a bit larger than a golf ball. I can't see doing that unless there are more plant to supply greens.