In general we should observe following factors for a career in writing in a horoscope:
a. Mercury--its strength in radix, navamsa and divisional charts. Associations to it by aspects and conjunctions in radix and navamsa charts.
b. The third house--its strength, the occupants and aspects to the house. The dignity of lord of third house in the radix and divisional charts especially in navamsa and dasamansa. The associations by conjunction and aspects for the lord of the third house.
c. The fifth house--its strength. The dignity of lord of the house in the radix and divisional charts especially in navamsa and dasamansa. The associations by conjunction and aspects for the lord.
d. Karkamsa Lagna---Please see articles A and B for combinations in the Karkamsa lagna for writers.
e. Third house from Mercury--The occupants and aspects to the house. The dignity of lord of the house in the radix and divisional charts especially in navamsa and dasamansa. The associations by conjunction and aspects for the lord.
f. Moon conjunct Neptune:
The influence of Neptune over Moon(mind) makes one capable of inhabiting skin of other people. The ability to feel how others feel makes one capable of creating many powerful fictional characters. This ability also helps one empathize with others.
Mercury is most important planet for careers in writing because when it is strong it confers the ability to articulate, narrate, create stories. If Mercury is dignified in radix and navamsa charts--i.e. it's either exalted or in its own rashi--it becomes a good combination for writing. Conjunctions and associations with other planets which are dignified also give writing ability to the native. Exaltation of Mercury in navamasa and dasamasa makes one an able writer. The association of Mercury with the lord of Dasamsa is also a good combination.
If Arudha Lagna falls in the third house or has Mercury in it or its lord is Mercury--it suggests a native capable of writing easily--though chart must be thoroughly checked for actual realization of such potential.
The third house in horoscope is important for writers because it's a house of hands-on learning, packaging and communicating. Stelliums or many planets in this house support writing. Aspect from Mercury or association of its lord with Mercury also support writing career. If the lord of the third house is well dignified in radix and navamasa charts--it's a good combination for a career in writing. Association of dignified Rahu with the third house or its lord is also a good combination for writing.
The fifth house is house for creative expression and celebrity. If the fifth house has Mercury in it or a lot of planets in it--it's a good combination for writing. The lord of the fifth house well dignified in radix and navamsa houses and/or associated with Mercury also suggest a good career in writing. Rahu in the fifth house suggests an ability to express and gain attention by writing.
The third from Mercury should also be observed. If it has a natural or temporal benefic then it's a good combination for a writer. The lord of the third from Mercury associated with it is a good combination for an author. If the lord of the third is well dignified in radix and navamsa charts-- it's a good combination for a writer.
For autobiographical writers--association of other houses/Mercury must be with the lord of the first house. Mercury in the first house or the lord of the first house in with Mercury or the lord of the first house situated in the third from Mercury suggest autobiographical authors.
For philosophical-religious writers--The ninth house and its lord must be associated with Mercury or situated in the third or fifth houses.
For mystical writers--The eighth and/or twelfth houses must have and their lords must be associated with Mercury or Rahu or the third lord. The lord of the third house in eighth or twelfth is a good combination for mystery writers. Similarly the the Mercury placed in eighth or twelfth house make for a good writer.
Strong Mercury associated with the tenth house or its lord suggests great public visibility and hence recognition for writings.
I have not covered many of the combinations but what has been covered must be good for starting your analysis. For better references and examples you might like to visit Barbara's page.