When I substitute yardage for a fq I will buy 1/3 yard for each one and it will be long enough to cut the pieces that you need, and then split the fabric on the fold and then cut the pieces out.
So from the 12" length of cut fabric you would have a 12x21 piece, from which you would
cut the 61/2 x 21 piece and then cut a 31/2x21 piece from this
half and from the other 12x21 piece you would cut the other
2 pieces then continue to follow the pattern as written. This
is the way I work with not having to buy a full half yard to get
the 18 inch length, yet still get the amount of pieces for the
blocks.