Hey guys, just wanted to ask a question here regarding custom records. How do I go about creating a custom record where there is a custom sublist for me to add custom line

I think you are asking how to get a custom record appear as a sublist on another record. If that is the case, on the definition of your custom record, add a field type with list/record of the parent record you would like your custom record to appear on. There is a checkbox on the field definition that says

record is parent

, check that box and you will see your custom record on the parent record,.

Does this apply to custom record as parent as well? What I want is to have a custom record appear as sublist in another custom record actually.

Yep, just create the field with that other custom record as the list/record, and click the record is parent checkbox