Choose a photo or a painting you have already done and simplify/abstract it by bringing out the dominant shapes and value. You can use any medium to render it again as an abstraction. Then abstract that abstraction, and so on, and so on! You can go as far as you wish with it. Have some fun!
Here are some examples I found on the internet.
There are all sorts of youtube videos to guide us… just search how to move from realism to abstraction in painting.
Experiment, take risks, and of course HAVE FUN!