How can I make sure message m ∈ Zp
It depends on what exactly you want to achieve. I don't know ORUTA.

But in general:
You have to convert your message to a numerical representation somehow. Then the message has to be short enough to be in Zp.

e.g. in Java:

    BigInteger p = [your p value]
    String message = "test message";
    BigInteger m = new BigInteger(1,message.getBytes(StandardCharsets.UTF_8));
    if(m.compareTo(p)&gt;0) {
        not good, your message is too big!
    }


Usually in signature schemes, you use the hash value of the message instead of the message, because a hash function has a fixed output size and for a suitable p you can be certain it is always &lt; p.

This is also quite simple, but if you say you want to compute a hash, it is probably not what you want to do?