I wanted to use the bag for carrying books, so I made some sketches about books and butterflies, which I used for the design. I drew the butterfly/books on a piece of A4 card and cut out the shapes using a craft knife. Then I sprayed a little glue on the back of the card and stuck it onto the blank bag.
I put some fabric paints out on a piece of acetate (a paper plate would be fine) and used pieces of sponge to stencil the design through the cut out shapes in the card. It's better to dab the paint on gently and repeat the application, rather than blobbing lots of paint on, as the paint could get under the stencil and spoil your design. When I had finished applying the different colours that I chose, I peeled the stencil off.
Then I sprayed some more glue onto the back of the stencil and stuck it onto the bag again, as I wanted to repeat some of the butterfly-books and fill in the bottom corners of the bag. I got a bit of paint under the stencil by accident this time, but it can be fixed afterwards with a paintbrush.
When I'd finished (both sides) and the paint was dry, I ironed the bag on the reverse, to fix the fabric paint and make it washable. Here's the finished bag again: