The apron edges need to be parallel for only a 1/4". Wool fabric is very mold-able and will take the shape you want when pressing. Starting at the top or bottom edge, fold the fabric at your chalk line and only concern yourself about the inch or two that you will be basting next. The top and bottom of the kilt should have about 2" of straight material so fold it parallel.

At a line, stick your needle through the fabric. Skew and stretch to make the thread line up on both sides, then baste. Don't worry about the puckering that you may see the pressing will flatten it out later.