Adding defaults for custom field - dependant on encounter medium civiCase/rules
depends on what you mean by simple instructions :-) As you probably know simply is subjective.........

If you mean: without programming, you can not add a new rule in CiviRules without programming. 
If you would like to program but find the instructions on the CiviRules cookbook https://docs.civicrm.org/civirules/en/latest/create-your-own-action/ too complicated I reckon you will need some training on CiviCRM customization as most of the terminology used is IMO fairly straightforward for CiviCRM developers?