I believe you are referring to UPSC CSE and not PCS Exam. For UPSC CSE you have 3 steps before success.
1 Prelims Paper
2 Mains Paper
3 UPSC Interview
You need to clear each stage meritoriously, followed by Final Interview. After Interview, Rank List is prepared and around 700-900 aspirants are finally shortlisted across India, for various Class/Group A Services, besides IAS, IPS, IRS, IRTS, IRPS, IFS.
For more information about UPSC Pattern do follow the link below
I would keep it very simple to you and in a short note you will understand the procedure. So here it is-
UPSC- It is conducted in three phases- a preliminary examination consisting of two objective type papers( General studies paper-1 and general studies paper 2) which is the civil service aptitude test and a main examination consisting of nine papers of essay type in which two papers are qualifying and marks of seven papers are counted followed by a personal interview.