I have a /custom_modules/db.js and am trying to require it from /routes/users.js but am unsure of how to do so.
You use ./ to signify local directory that you are in, and ../ to go back a directory. Therefore:
require('./../custom_modules/db');
Should just use ../
Example :
var db = require('../custom_modules/db.js');
Just insert a leading / to give an absolute path:
require('/custom_modules/db.jpg');
The rules for resolving modules path are detailed in the modules documentation.