Indian Engineering Services (IES) is a national level exam which is conducted every year by UPSC. 

The following tips you can follow:

Firstly go through the syllabus of each paper of IES exam which you get in detailed notification of IES released by UPSC.

Go through basics of your core subjects

Go through previous year paper of IES exam which can be downloaded from UPSC website and try to solve it so that you will get aware about your weak and strong areas.

Study the weak subjects again and again and try to gain confidence by practicing more questions on that topic.

Good Luck!