Hi members ... I need some assistance ...

I'm developing a Form using RSFormsPro [ver 1.52.4] with Joomla 3.6.2.

I have a dropdown list auto-populated by a custom db table [countries] within the Joomla database. This is working.

On the same form I have a number of fields corresponding to the fileds in the countries table.

I need to populate these fields from the same custom db table [countries] based on the selection in the Dropdown list.
On the same form I have another Dropdownbox that needs to be updated depending on the selection in the first Dropdown list.
The RSForm form will be submitted with these populated fields.

I know I cannot do this via PHP as it would mean refreshing the form ... I have seen examples where JavaScript is used but cannot connect the dots .. ... I know I should place some JavaScript in the JavaScript & CSS section of RSForms and relate that to the form fields to be populated ... I don't know how ... or how to reach the database from JavaScript & CSS section of RSForms... I can get the selected Dropdownbox-value as variable using JQuery, but then i have no idea how to continue.

Any help will be most appreciated ... I don't need to copy anyone's code [although that will be the easiest ] ... I just need a step by step explanation on the process ... I shall post my code if this is needed.

My competency : PHP medium ... JavaScript/ JQuery good ... MySQL good.