Here is some useful documentation.

You're going to want to make a class that extends %CSP.REST and set up an application that uses that class as its dispatch class. You'll have a URL map in that class to tell IRIS or Cache what to do with the request. Depending on your specific application, you might also want to get familiar with using %request and %response in that process.

When you're projecting a list of a class to XML, you usually have a parent element for each of those objects, then a separate element for each of it's properties. So I think what you'd need to do would be something more like:

<Results>
    <PersonIDs>
        <PersonID>
            <PersonID>1000000</PersonID>
        </PersonID>
        <PersonID>  
            <PersonID>1000001</PersonID>
        </PersonID>  
        <PersonID>  
            <PersonID>1000005</PersonID>
        </PersonID>  
    </PersonIDs> 
</Results>

So that the outer PersonID element indicates your PersonID object, and the PersonID inside of that is the PersonID property of that object.