The human anus is typically pretty stretchable. If you are an anal virgin I would start with fingers, once you're comfortable move on to butt plugs starting with the smallest one. Slowly keep stretching with a lot of time and a good about of lube. Eventually you should be able to take the larges butt plug. If your boy is very large or has a large knot you might have to continue stretching your anus with a dildo. Stretch as much as needed.
I've heard horror stories of people injuring and tearing their butthole when not being careful, and were unstretched. This is what I hear works from other zoos. I myself only have interest in female animals.