Stirring with knives?
You need to incorporate the butter into the flour such that the butter is in hazelnut sized lumps, without melting the butter. If you chop it that small on a board it will soften as you handle it. So you have to do it in the bowl.

You could buy a pastry cutter/dough blender but to be honest they are a pain in the wrist. If you have a food processor, you have a perfect pastry making tool - just blitz the butter and flour with a couple of pulses, then dribble in some cold water, blitz again, and repeat until you have a coherent (but not too sticky) dough with the aforementioned hazelnut sized lumps of butter mixed through.