It should be pretty much the same. If you can sign a CSR with RSA you can sign with ECC. The exact same data is signed, what changes is:
The signature algorithm should be the right OID (e.g. ecdsaWithSHA256)
The signature value must be the ECDSA signature encoded as specified in SEC (i.e. an ASN.1 sequence of the two integers in the signature)
The public ...