Painting plaid pants is not the easiest thing to do, it takes a lot of focus.
For the blockin of the plaid pants I just painted in the squares and left the major lines as the gesso color. For this session I put a glaze of paint over the entire pants with correct value changes for the folds. So the old lines were showing through the glaze a bit yet I got the structure of the cloth down. The rest was all about putting in the plaid and making sure that I got the value changes correct for each line as it roamed up and down each fold.