I drill out the handle, in several steps, to the width of the tang (the shortest distance across). Then I take a chisel and widen the part that needs to be wider to fit the tang. This produces a rectangular hole, tapered because of the different drill sizes used at different depths.
You can use a caliper to check drill depth against the tang. Almost all calipers have a depth rod that sticks out when you open the caliper. This way, you can fit the drill size to the tang at the right depth.
Tap the handle to drive it onto the tang. The first time it won't fit but shake the chisel and you'll get an idea where it's hitting. Use your chisels to widen that part. Keep doing a trial fit until you can drive the handle on without breaking it.
If you have trouble removing the handle after driving it on, hold the chisel by the blade and tap the handle against your bench. Tap one side then the other. The handle will come off.
Hope that's clear enough. It seems hard to explain.
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