Stern identification scheme to signature scheme
In order to convert a 3-move id-scheme to a digital signature you have to replace the request (or the challenge) by the value of a secure hash function applied to the message (this method is called Fiat-Shamir method). 

In Sterns's id-scheme you have to choose a random element from the set $\{0,1,2\}$ for your message. That is $H(m)\in \{0,1,2\}.$ For  instance if you have $H(m)=0$ then the signature of $m$ is $(c_1,\Pi).$

EDIT : Before you convert it, you have to run the id-protocol many times, say 64 or 80 times, and then apply Fiat-Shamir method.