Add caulk to piping bag
Using a caulking gun and your desired caulk, fill a plastic disposable piping bag with half of the caulk. This should be enough to do one cake wedge. I was able to pipe two cake postcards with one tube of caulk, but I recommend putting only half of the caulk in the bag at a time. Also, I tried decorating my cake wedges using the caulking gun but it was nearly impossible to control.