It is difficult to prepare only at home. You have to gather knowledge on so many topics of so many subjects. Specially choosing subject is difficult too. One need guidance for all such stuff. Join any credible academy and get proper guidance from where to start.
First, you take proper guide related to CSS from experience person.
Stock Dogar books (Dogar brothers Publishers) at home n read em daily plus start following current affairs with dedication.
Karachi University have 3 month basic training course. All compulsory subjects are taught along with availability of books.
Check CSS Forum website (CSS Forum – CSS Examination Preparation & Civil Service of Pakistan). Select subjects and write most repeated questions of your selected subjects. Read newspaper and daily write two pages on any topic of interest in English, first check it yourself, than give it to any expert to check it.
Paper attempting technique, keep proper space between words and lines. Attempt paper on blank sheets (without lines).
Practice old papers as much as you can, write answers as if you are in exam with time limits. Get your answers assessed by any CSP officer.
Jahangir Book and internet can help a lot in prepartions.
In Karachi academies are not good but in Lahore and Islamabad I have heard that academies prepare students properly.
The above discussion was done on Facebook group.