How do I sharpen a Japanese knife on a whetstone (freehand)?

Freehand whetstone sharpening is a learnable skill, not a knack. Wet your stone the way it needs (older soaking stones sit in water for a few minutes; splash-and-go stones like the Chosera just need a spray and can be damaged by long soaking), set the blade at roughly a 15 degree angle to the surface, and use light, consistent strokes across the full length of the edge until you feel a burr form on the opposite side.

Once you have raised a burr along the entire edge, flip the knife and repeat on the other side, then finish with a few very light alternating strokes to remove the burr. Counting strokes is a common beginner mistake: feel for the burr instead, because every knife and steel responds differently.

Our K&S take: start on the in-stock Naniwa Chosera 1000 (or a 400/1000/3000 ladder for chip repair through to refinement) and keep the stone flat with an Atoma diamond plate. A sharp edge on a dished stone will not hold a clean angle.