Fold your fabric selvedge to selvedge, start at one cut edge & match the plaid line & place a straight pin in it, move to the next plaid & repeat across the cut piece. Once it is matched up, pick it up & let it hang out, lay it down & see if the folded edges are matched, if not, then repeat the procedure on that side as well. It does take time, but curtains made with a plaid on a slant isn't my idea of fun either.