Condition to allow only the user stored in a custom field to execute a transition
To add Check user in custom field condition, go to the transition of your workflow, switch to Conditions tab and click on Add Condition button.
Select JEP - Check User In Custom Field and click Add button
All User custom fields will be listed in the upcoming screen, select the one you want and click on Add button
That's it, the condition is added to the transition.
Please do not forget to publish the workflow.