You can trigger Work Relay Action or Work Relay Action Group from your Salesforce Process or Flow

• Create an Action/Action group you want to execute from the Process/Flow. See details at Actions/Action Groups.
• Add new Action to your Salesforce Process/Flow with type set to 'Apex'.
• Select 'Execute Work-Relay Action' as an Apex Class.
• The Action/Group ID or Name - pass Action/Action Group Id or Name. 
• The Source Record ID - If your action have source object you should pass source record Id.
• The Initial Variables - pass variables to you Action in JSON format.

In Salesforce Flow make sure to create variables to pass them to the action as parameters: