I learned horary mostly just by reading the available texts on the subject. In my opinion, none of the modern texts do it justice as just about each one misunderstands some fundamental concept or technique.
The best texts on horary available right now are going to be William Lilly's Christian Astrology, Guido Bonatti's Treatise 6: On Questions, al-Kindi's Forty Chapters, and the Book of the Nine Judges.
Aside from that, the only thing you can really do is practice by trying out the techniques you read about on charts, but there are some substantial barriers to finding quality charts to trying to learn anything from.
There are also courses you can take on horary, but very few are any good, so you have to be careful with that.
