You can use a shamrock-shaped cookie cutter or a cut-out or you can find a template online to trace onto the piece of cardboard. The size of the cardboard will depend on how big the item is that you're tracing. You'll want to keep it small, however, so that your toddler will be able to color it completely without losing interest.
Of course, if you can draw, by all means freehand it. Another way to quickly create a clover is by cutting out hearts and then placing them together with their points touching.
Some other shamrock templates:
Once you've selected your template and have it cut out, trace it onto the cardboard.
