These options are available from the Flow Builder Canvas.

1. Select the Process or Connector you wish to modify.
2. Click on Rules / Actions in the bottom of the Properties area.

1. Click the green plus sign to set the rules that need to be verified before the selected item continues.
2. Name Your Rule.
3. Provide an error message that will be displayed if the rule doesn't pass.
4. Add Conditions. Rule Conditions work like the Start Conditions at the Flow level but are processed at the sub-level within the flow (the process or connector)
5. Save Your Rule.

It is possible to create multiple Rules for each Flow Step