Modify phpBB forum during registration

Perform well in this (in terms of quality code, delivery time and limited bugs), you will get first refusal on future projects. The forum based website needs an alteration. It has an unaltered phpBB forum v3.2.5 installed. I would like the successful freelancer to provide me with instructions on what to change in which files to do the below (rather than giving out access information - this is to protect both parties). I am computer literate, but am not expert enough with PHP, JS, CSS or HTML to do this work. What needs doing: - During registration process, two additional fields required: Both dropdowns. Dropdown "A" determines the list of Dropdown "B" e.g. "A" {"Town", "City", "County"}:"City"; "B" {"London", "Manchester"} or "A":County; "B" {"Kent", "Surrey"} - Upon clicking submit, it will redirect to a PHP page (yet to be designed) that will be an extension to the registration process i.e. registration shouldn't complete yet.