Sentence:  To <WSD>fell</WSD> seam allowances, catch the lining underneath before emerging 1/4" (6mm) ahead, and 1/8" (3mm) to 1/4" (6mm) into the seam allowance. 
Meaning of the word marked with <WSD> tags:  To stitch down a protruding flap of fabric, as a seam allowance, or pleat.