First choose a name for the rule in the input form. In the following, you can define whether one or multiple conditions have to be fulfilled to trigger the action(s) of a rule. The conditions (If ...) can be defined based on the Counterparty or the payment Description. With "Add filter" you can add filter rules or delete them by click on the recycle bin symbol.
Finally, you can define the action to be taken if the rule applies (next to "Then ..."). If you choose "categorize", you need to select the corresponding category. If you choose "mark" you can define whether the transaction should be included in or excluded from the analysis. By clicking on "Add action", both possible actions can be combined for a rule.
Please note: custom rules are always applied for a transaction after the automatic assignment that is done by the system.