I have the instructions here and Step 8 says:
"Fold on a Bottom Square line, RST, as shown. Pin from bottom to top so seams interlock. Sew, easing as necessary, from top of rectangle to bag bottom-this will force any "discrepancies" to bag bottom. Tip: Check corners-if a seam "pops" out, carefully clip excess and restitch corner, encompassing seam."
Hopefully this is what you were looking for?