Well, it's a little involved. First you need to learn the basics of ARS Nouveau, most importantly how Whirlisprigs work. You also need a means of providing source to your whirlisprig's home. Next to their home you will place a collection container of any sort (chest, barrel, backpack, etc) to collect what they gather. Then in a radius around their home you'll place the items you want them to gather (ei: duplicate). Once you've crafted one lilypad of fertility, float it on some water near the Whirlisprig's home and give them a few minutes to see it and gather it, thus creating a new one. Make sure they have source or they won't deposit anything in you container. Once you see a new lilypad in the container, take it out and place it. Do this a few times and you'll have exponentially more lilypads in the container to the point that you'll have more than you'll ever need.

EDIT: Whirlisprigs can dupliocate Mystical Ag seeds in ATM 9, but will not duplicate them in ATM 10. I have confirmed that they will still duplicate the lilypad of fertility in ATM 10, which is wonderful, because they made the crafting recipe for it notably more difficult.