Step 2: Cut out the pieces
- 2 side pieces
- 2 bottom pieces (the bottom is double thickness)
- 4 end pieces (or 2, if you want to leave out the inside pockets)
- 2 side pocket pieces
To cut the end pieces, draw (directly on your fabric is fine, or on a piece of newspaper if you prefer) a straight line as long as the width of your plastic piece plus an inch for seam allowance. This will be the bottom. Now draw the sides and top as part of a circle, or other bulgy shape - I like to make it slightly triangular - at a size that looks like a good one for your needs. Measure the circumference at half an inch in from the edge (for seam allowance!) and write it down as you'll need to calculate the side piece sizes from it. Mine was about 40" (precision of an inch is plenty).
Using this circumference measurement and the size of the bottom plastic, if you have a restricting one, calculate the sizes of the fabric pieces to cut for the bottom and sides as follows.
For the bottom, your sewn measurement should be 1/4" greater all around than the plastic reinforcing piece, to account for its thickness. For me, this worked out to 10-1/2 x 17 inches (I cut the plastic a little shorter than I absolutely had to, by accident).
From my 40" circumference, I then subtracted the 10-1/2" for the bottom piece, and another 1/2" for the width of the zipper, to get a difference of 29". This distance comprises both sides, each of which must therefore have sewn dimensions of 14-1/2" x 17".
The handles will lay at about the thirds of the bag length, putting them about 6" apart. So that's the width of my side pocket pieces, which will be sewn between the handles. Accuracy in the width is less important here as the straps are 2" wide and will cover the edges.
The cut pieces should be one inch bigger in each direction than the sewing dimensions, for seam allowance. So my final cutting dimensions are these:
2 @ 11-1/2 x 18 (the bottom pieces, remember it's a double layer)
2 @ 15-1/2 x 18 (the sides)
2 @ 10 x 6 (the side pockets, which I just decided would be 10" tall based on eyeballing it)
Whew! Measure again to doublecheck yourself before you cut. Sewing as well as carpentry uses the advice "measure twice, cut once".