In the wild, White's lay eggs from November to February, which is their local Summer. In order to get them to breed in captivity, you have to fatten them, induce aestivation, bring them out of it, fatten them again, and use a rain chamber to induce mating.

First thing to do obviously is becoming very good at keeping them healthy. While your frogs mature (it's recommended to wait until they are 3 years old before breeding), research and read as much as you can.

You can probably get a used copy (out of print books) of "The General Care & Maintenance of White's Tree Frogs" by Philippe De Vosjoli or "The Guide to Owning White's Tree Frog" by Coborn, at Amazon. Also, can search the net for articles like this one: Breeding White's Tree Frog; good luck !