Fill tub with a couple of cms of water. Add food colouring (paint will also work but make sure children don't suck up the straw instead of blow) and some washing up liquid. It's trial and error to get the amount right depending on size of the tub (the larger the tub the more food colouring needed).
Use your straw to blow bubbles into the mixture. Get the bubbles as high as you can.
Gently place your paper over the tub and catch the bubbles.
You will then have a print of the bubbles. You can layer bubble prints over each other and try different colours. Experiment and have fun!
Bubble prints are great for making underwater pictures.