Generate Derived Key from Shared Secret (ECDH-ES) - ConcatKDF

I have used the https://github.com/ricmoo/GMEllipticCurveCrypto to generate a Shared Secret using ECDH Algorithm.

I now have to Generate a Derived Key using the ECDH-ES algorithm as mentioned here: https://www.rfc-editor.org/rfc/rfc7518#section-4.6

I had highly appreciate if someone could guide me in the direction of generating a Derived Key