It's just like the DEF tanks on 18 wheelers. I buy a 10L jug of it from Walmart for something like $10. In my trunk there's a panel you remove and under it there's a small cap very similar to the gas cap. Remove that, hook up the DEF bottle hose (the bottles come with a 12-15" corrugated hose) and very slowly pour it in. You don't want to spill that stuff, it's nasty not because it's urea, but because when it dries it kind of crystallizes and makes a real bloody awful mess.

Replace the cap, replace the panel cover, close the trunk and you're done for another 9-15mos.