Haven't seen a long DME arc in Europe for donkey's years, they probably do exist but are petty unusual. The few that one does see are such short segments that the technique below hardly has time to work.
On a long arc technique is to fly 'thrupenny bit' shape, you can't do it in a continuous arc.
Pick a heading that will take you a little into the arc, (perhaps .2 to .4 miles on a 10 mile arc) fly out through the circumference by the same amount, turn to cut inside the arc and back out again. Repeat as necessary.
Cumbersome procedure.