There are several rituals that allow one to transform into a Unicorn. The most dangerous is to reincarnate as a horse and believe in the Afterlife while still a horse. As you can imagine, this is a very difficult task to accomplish, which accounts for the rarity of unicorns.
There are other methods. Steadfast prayer combined with learning as much as possible about horses, mythology and horns. Watching My Little Pony daily is also highly recommended, if you go this route.
I heard that Dracula's old castle once contained a book on this subject matter but when Scholomancy began to be hunted down by the Dominican Friars in 1722 this book, titled _How to Transform Into Magical Creatures_ was hidden away and not seen again, although there are rumours some copies survive.