for a tote bag with a flat bottom, I use plastic like from a(old) place mat , then you dont have to get it out of the bag when you will wash it in the machine. I sandwisch this plastic between 2 layers of fabric, sometimes I (also) iron very stiff pelloni on both the layers of fabric and than sew the 2 as one unity(bottom), that also works fine. Hope you understand what I mean, I'm from Belgium and englisch is not my useal language, normaly I talk and write dutch.