This was one of those projects which took me a little longer than I thought it would! You could throw it together very quickly if you don't make my French style seams - which I added to strengthen the bag for those heavy spuds - but I'm pretty confident that I can put almost anything I could fit in to this bag! I say French style as they're not technically French seams, but something similar. The seams also have the added benefit of hiding all those raw edges.
Anyway, here goes. You will need:
A sheet of A3 paper (to make the very simple pattern).
A fat quarter of a (wide) meter of the main fabric (the polka dot).
Just under half of a fat quarter of a yard of the second fabric (my 60's Floral design). Or equivalent quantities of each - this bag is good for using up some of the fabric stash!
Take your piece of A3 Paper, pin the whole thing to the main fabric and cut out x 1. Remove the pattern then cut this piece of fabric in half lengthways so you have two identical strips (see pieces on the left of the image below).
Fold the piece of paper into thirds (as if you're folding a letter), and cut one third off. Then cut 2 x the larger (square) piece from your main fabric and 2 x the smaller piece from your second fabric (see below). You should have 6 pieces in total.
Pin the smaller pieces (of the second fabric) to the large pieces and stitch together (wrong sides together). Press.
Trim the excess from one side of the seam allowance (I chose the darker fabric so it wouldn't show through on the outside of the bag).
Fold the larger side over and around the smaller one to conceal it. Press and pin in place.
Top stitch this in place as close to the edge as possible.
Repeat for the other side of the bag.
Fold each of the strap pieces along the long edge, pin and stitch.
Turn them the right way around and press.
Then top stitch along both edges to help them to keep their shape.
Turn the tops of the main bag pieces over by about 0.5cm and press into place.
Lie right side up and pin the straps onto this top edge (they should overhang by about 0.5cm). Measure to ensure they're equally positioned - mine were 4cm in from each outside edge.
Stitch these in place 1cm from the end of the strap (0.5cm from the end of the bag top).
Turn the bag over and fold the edge in.
Then fold it again so that the straps lay across the wrong side of the fabric (the top edge of the bag should've been turned twice).
Finally, flip the straps back the right way, retaining the two folds on the bag top.
Pin firmly in place, right along the top, then top stitch as close to the edge as possible.
Place the bag pieces together then carefully position (and measure!) the straps for the second side. Once you're happy, repeat the steps above to attach the straps.
Next, turn the bag so the right sides are facing one another, then pin and stitch the bottom edge.
Make the seam in the same way as the first one above, so the base of the bag is nice and strong. Then stitch the side seams together using the same technique.
Give it a good press, and you have a bag!
It's easy to vary the size of the bag - the easiest way to make a larger one would be to use a sheet of A2 as your pattern (check the length of the straps but you might then be able to get it onto your shoulder...). Alternatively you could make your own pattern to whichever size you like - just make sure you start with a rectangle.
You also could make the bag even sturdier (and prettier) by adding a nice contrasting lining.