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.
There are many Green Porno videos in a series titles “Seduce Me: The Story of Animal Attraction,” and many of them can be 
viewed here. I recently became aware of these videos, while viewing an exhibit on animal sexuality at the British Natural History Museum in London. At the entry of the exhibit, a sign warned that parts of the exhibit were frank and explicit. Some of the parents nonetheless walked in with their small children, and were aghast when they saw the section of the exhibit that included these videos and explicit videos of copulating bonobos). I’ll let you be the judge on how you would rate these. I found them to be imaginative, funny and scientifically informative. I’m also posting (below) the video concerning snail sex, one of the videos the Natural History Museum was playing when a man and woman scurried away with their children:
Many years ago I decided that the traditionally accepted definition for the length of a cubit was grossly off; in order for an ark of 300 cubits in length to handle all the species, 18″ couldn’t work.
I made some very generous assumptions:
…number of species (disregarded the creationists’ lame claim that species divergence came after the deluge and “type” drastically reduced the logistics) – around 60,000 vertebrates and 1,000,000 invertebrates like insects, spiders, worms, etc.; disregarding how the species from the other continents got there…
…allowing for no spoilage of the vegetable matter (a reach, but then look at what we were asked to believe), sufficient quantities to last all the vegivores for whichever biblical version of the story you choose to read – 40 days/nights or the 150 days…or seven months…or ten months
…disregarding specialty diets like eucalyptus or bamboo
…sufficient meat animals to feed the carnivores – and the requisite vegetable matter for them; serious multiplier considering how much the largest carnivores need to survive
…disregarding the natural rapid reproductive cycles of the chosen animals (but not the feedstock)…lots of rats and rabbits running around would not be good
…sufficient water storage for when the rains stopped, disregarding spoilage, of course
…disregarding the physical impossibility of feeding all the animals
…allowing for the rains to wash away waste only covered the first month and a half; I stopped trying to figure out how to manage it after that
…disregarding the muscular atrophy of being penned for 10 months
…allowing for discrete holding areas to keep the animals that were to repopulate from eating the others that were supposed to repopulate really added to the internal volume
…disregarding that the largest wooden keel ships that were seaworthy and stable were less than 300 feet
…disregarding the elaborate structure and additional building material to support the weight of all the different species in their different areas
…considering the tools and techniques available drove the minimum size of the containment areas
…disregarding the supply logistics for the lumber necessary
…and I don’t remember the rest – it was like how I used to code the fun stuff in the old days: I’d get a mostly working product…then add features, and features, and documentation, and tweak, and hash it…
Well, the length of the vessel turned out to be several miles long, a couple wide and the number of board-feet required would have stripped the Sahara Forest bare, so I’ve had it in my head ever since I gave up that a cubit is actually something on the order of 60 feet (though probably much larger).
But then that made the other Ark that was raided in 1981 pretty huge.