You will need to use an IF activity for your condition, then place a THROW activity inside the IF. Since this will be a BusinessRuleException, you can use this in your Throw:
Then, in your CATCH of the Try/Catch, use an Assign to store the exception to a BusinessRuleException type variable inside the BusinessRuleException type. Then, in your Main flowchart, you can use a Decision or IF with condition
businessRuleException isNot Nothing and Log the exception and/or Throw it again if desired.
Please also check out the REFramework under Templates found in Studio. This could give you some good ideas on error handling as well.
Hope this helps.