The easiest way that I have found to do it, is to get a big piece of cardboard and cut it to fit the flare. That way you can make adjustments as needed on the cardboard, until it fits the flare perfectly. Then just transfer the cardboard to the side of the Jeep and trace the edge that needs to be cut.
You can also transfer the holes for the mounting hardware this way as well.
Works great to make a pattern, and if you do it correctly, you can use the same pattern for the other side just have to flip it around and it will line up.
Good Luck, it's a great mod to gain a little extra clearance.
'87 YJ, 4.2L built and bored, 8" of lift, 8.8 rear, AX15/NP 231J, Tom Woods drive shafts, Flat fenders, 12,000LB winch, Custom bumpers, Rattle can paint job. Never completed...