how do I create a Custom webservice using Python-eve?
If it was in Flask, I would only put the route decorator before the function and inside the function I could do whatever I wanted (call another service, run a python function, etc).
With Eve, I define de resources (domains) and it processes put, get, etc by itself using the database.
Should I just decorate a function like I would do with Flask?
You could also register a Blueprint like you would do with any other Flask application. That's what Eve-Docs is doing. You might want to check it out.
Eve is Flask (in fact, it is a subclass of it) so, in general, whatever works with Flask works with Eve too.