Parenthetically cite the last name of the author, date of publication and page number, letting "n.d." stand in for the unknown date of publication. For example:
(White, n.d., p. 19)
Cite directly after a quote or paraphrase from the source material. For example:
One of the best ways to cook fish is "with a little bit of garlic, but not too much" (White, n.d., p. 19).
Organize your reference list entry as usual, replacing the date of publication with "n.d." For example, your reference list entry for a non-periodical website with no date of publication listed might appear as follows:
White, S. (n.d.). Tips for cooking fish. Retrieved from http://www.cooking.com/fish/recipe1653.htm