I didn't do anything out of this world, my plan of action is what everyone does for each and every topic. To prepare any topic in UPSC, following three things need to be done -
1) Read and revise relevant study material
2) Do PYQs religiously, of both pre and mains
3) Give Tests of those topics, both pre and mains
My sources for Ancient and medieval -
1) Old ncerts - Ancient India by RS sharma and Medieval India by Satish chandra
[Non Ncert editions of above topics by same authors is available but its too difficult to read and comprehend, while NCERT version is easy to read]
2) Themes of India - all three parts
3) Nitin singhania's 6 chapters for art and culture - Architecture, Dance, Music, Painting, Religions of India, Bhakti and sufi movememt in Annexure
[These are major chapters needed to be perfected, rest can be read as per convenience]
4) PYQs of prelims and mains religiously
5) Sectional tests of any coaching, I did VisionIAS, simulators, daily mcqs on Insights
Other resources referred by aspirants -
1) Lucent - I couldn't do this as I found it too summarized and had only facts to mug up
2) Art N Culture ncert - I did this 2-3 times over my preparation span but couldn't ever recall whatever I read in it.
In UPSC, especially for static, it's not about what all you have read, its about How many times you have read.