Can refer to the steps described in 2 posts in this previous topic-thread here and here.
Asking around before getting involved in some scheme that one doesn’t fully understand.
Good timing! 
You can start by reading about Bond Yield
and then this chapter on how one can calculate the yield of a GSEC bond,
and how interest payments are received from a GSEC.
Apart from this,
just FYI here’s a quick summary of a few scenarios
to extract additional returns from investing in bonds through the secondary markets like NSE/BSE.
(in addition to receiving periodic interest on them)
( Not that you should explore them right away,
since you wish to invest in GSECs mainly for the “purpose of pledging for Trading margin”.
But, thought of sharing here, these few other possibilities surrounding GSECs (and bonds in general), since we are on the topic.)