The usual tools are....

The Tee handle.

A cutting head with the degrees angle you want the cone to be.

A precision drop-in plug gauge to measure the cone for proper dimensions. Note that you cannot eyeball the cone for proper size.

A brass lapping head and fine valve grinding compound.

You probably should also have a 90 degree cutting head to set the barrel-cylinder gap.
This is far superior to trying to file the rear of the barrel and keep it perfectly square.

Brownell's is the go-to source for the supplies.