1/ You have installed the Joomla Custom Fields Search package.
2/ You have published the Module.
3/ You have created at least one custom field. (Follow this tuto if you don't know https://docs.joomla.org/J3.x:Adding_custom_fields)
For any custom field, you can create a search entry into the module.
New Custom Field
Let's say that we have created a CF named Price to add a price to article.
New Custom Field Search element
Create a new Custom Field Search entry
From admin panal, go to Component > Custom Fields Search
Create a new element and fill mandatories fields :
Label : Search field label in the module form.
Description : Search field description in the module form.
CSS Class : CSS Class, if you want to add client side validation (https://docs.joomla.org/Client-side_form_validation)
Custom Field : Lists all available custom fields in Joomla content.
Display Mode : The way you want to display the search field. You can choose custom and write your own display mode using standard JField (https://docs.joomla.org/standard_form_field_types) or custom one (https://docs.joomla.org/Creating_a_custom_form_field_type)
Query Operation : Choose the corresponding search query operator you want to use.
Module : Select the module instance(s) in which you want to display this field. If empty, the field will be displayed in all CFS search module.
Status : Publish state
CFS module rendering
Now you can see the Price search with the picked options :
Display : Range (Min/Max)
And if you make a search it will search with the choosen picked option :
Query Operation : Between