If you don't want to go to the expense of buying cutters just for this, I'd suggest a cheap at of metal heart cutters.
Where I am in the UK, the proper peony ones cost upwards of £22. A lot of money to try a new flower. I bought a £5 set of heart cutters instead and, using some needle nose pliers, bent the wide end of the hearts into a wobbly end, like the frilly peony cutters.
I was going to buy the proper cutters if I liked how the flower turned out but honestly don't need to, the fake ones are great!
You need to lay the paste on top instead of cutting onto a board as the metal won't be quite flush one you fiddle with it, but if you do the trick of leaving the flowerpaste to dry for 5 mins, then flip over so you cut from the soft side to the dry side, you get nice clean cuts.
If you Google diy peony cutters you'll see what I'm poorly trying to explain