According to the Bible, Noah was ordered to take one male and one female of each species. This part of loading the ark seems easy, at least conceptually.  Just gather up one male and one female of each of the many thousands of species of critters before beginning the journey.

7:2 Of every clean beast thou shalt take to thee by sevens, the male and his female: and of beasts that are not clean by two, the male and his female. 7:3 Of fowls also of the air by sevens, the male and the female; to keep seed alive upon the face of all the earth.

But there is a problem: many critters don't mate in the "traditional" way, using one male and one female. Luckily, there's help on the way: a reenactment of the boarding of ark.  This delightful and informative "Green Porno" video features Isabella Rosellini.