Before we begin, I would suggest going through some of the good books on Medical Astrology. Medical Astrology, A Rational Approach is a good book, so is Jyotish Aur Rog (ज्योतिष और रोग), written by Pt. Kisanlal Sharma. This book is published by Manoj Pocket Books and should be available everywhere. The language and style of writing is very good. These books have a table for planets/rashi/bhava in relation to most common diseases and it makes for a good reference.
This is how we should start the study of a horoscope for diseases. Look up the reference body part, study the association with an agent and then predict diseases during the dasha period of all the planets involved.
Reference body part:
Understand that organs and body parts are referenced by a house (bhava), rashi and a planet.
The planet representing an organ in their natural tendencies are the Karakas. The planets that represent an organ through house ownership are the Lords.
Bad aspect/association of both these type of planets shows up as a disease.
E.g. Sun is the natural Karaka for heart. In a horoscope, 4th house represents heart. Hence to judge for heart disease, we should look at influence on both Sun as well as lord of 4th house. If one of these factors is strong, the effect of the disease are not likely to show up. If both are weak, then the problems will be worse.
Agents causing disease:
In a horoscope, association of the Lord of 6th house with any of the planets will show up as a disease. If both the Karaka and Lord are associated with 6th lord, then problems will be magnified.
Position of planets in 6th house in the horoscope causes problems.
The planet Mars signifies wounds and injuries due to accidents, etc.
Rahu is responsible for suddenness.
Saturn represents chronic diseases.
Please note that association and aspect by an agent is essential. Weakness of the Karaka/Lord is important. General Weakness, but not amounting to disease should be looked at very carefully. If a planet is debilitated or is associated with other inimical houses, e.g. 12th and 8th, it will not cause disease by itself. However, it will signify general weakness and lack of development for that body part.