Evaluation of ERPNext help for quick prototype

Can you please give me a quick example
of how i can use custom tables for direct access to db to get info
and a RPC to deliver me the JSON. What would be the best practice using
the using an already created but empty application / module and a new site. I am
trying to make a short prototype with not much time and i am struggling
with the structure. I tried the RPC example here https://frappe.github.io/frappe/user/guides/integration/rest_api.html but i can’t get it to work.

Thanks in advance

@WhiteKarma what did you try so far? Did you get an error?