It is highly recommended that you submit the bug via Core-Exile's Bug Tracking system. You can access it directly from the launch screen, via CE Bugtrack, from the top row of links, or by clicking the red lady bug icon while logged into the game.
When submitting from within the game, the bug submission interface is pretty simple and self-explanatory.
Submitting through CE Bugtrack involves going to "New Ticket", beneath the "All Tickets" category on the left hand side, then clicking the "Add Ticket to this Project button". The drop-down should only have Core-Exiles as an option. Then simply fill in the form as best you can and submit.