To make D shaped rings you need a D shaped mandrel.
Or do you mean D section wire?
So first of all to make D section wire, there area no of ways.
Excluding special tooling for mass production, a one man band will
usually take round wire and using a flat faced hammer, forge it down
to shape in a half round grove in a piece of steel.
How to make one of these is another lesson.
Square section wire is more difficult to get to the D section this
All the Durston mills Ive seen and on mine has the roll shafts
extended on one side to take extra rolls.
Mine came with a pair, one flat and the other with 3 half round
Durston will supply you with these rolls.
Will make your life a lot easier.
Dont use copper sheet in your mill, you need to use the hardest
sheet you can find. this will enable you to use a thinner sheet.
Use spring steel, say, 1/16th in thick.
As to forming a circle with your half round wire on a ring mandrel.
a soft leather mallet of what you should use.
any hammer harder than the D section wire will mark the wire.
Common sence really.
Dont be afraid to hit it! And experiment.