Compare. Pick any one thing to write about and make as many comparisons as you can. This is good for helping you to develop original metaphors that you can later use to create extended metaphors. It doesn’t matter if you start with a few cliches, because once you get those out of the way, you will have to start thinking of new ways of comparing things. If you have a hard time starting, start with obvious things, then move on to the less obvious. It’s okay to stretch the comparison. You’ll be stretching your imagination as well!