in the original paper of Ring-LWE (https://eprint.iacr.org/2012/230.pdf), page 18, definition 3.1, they take a cyclotomic ring of integers $R$, and a prime number $q$, and then multiply an element $a$ from $R_q=R/qR$ with another element $s$ in $R_q^{\vee}=R^{\vee}/qR^{\vee}$.
My question is how they can define such multiplication?