It's fairly easy, but it took me a few hours to get the hang of it. In short it comes down to it that every field in the custom fields you use can be added to the main search bar or be used as a custom search field.
You need advanced filters installed and have set the searchbar set to 'main searchbar and advanced filters'.
If for example your site is about classifieds in the wordpress admin select 'classifieds' and then select 'settings. Now in the tabs on top click 'search'.
On the left you see all fields you can add to the main searchbar.
But there's a snag.
If you add a field to the main searchbar it is right there, always in view. If you want a specific field as an optional element to search by you need to click that field once it is on the right (so once you added it tto the search) and on the left the details will open. Now you see something like 'main search bar?'. Deselect that. Turn it off.
Below you see 'search operator'. The options ar AND and OR. AND is used when a a visitor can select more than one ite from a list. Like 'red' and 'yellow' when looking for shoes. That will result in showing all shoes which are both red and yellow. The OR is the other way around: it will show shoes which are red, yellow or red and yellow.
The INPUT (also a field in this form) lets you select how people can make a choice.
It's the basics, getting to know them is a matter of laying around with it all a bit.
