The first thing you need to do when designing your own automation workflow is to determine the type of Trigger that will set your automation in motion. There are various types of triggers available - the "When any custom field value is changed" trigger, as implied by its name, reacts by setting your automation in motion when the value of any of the Custom Fields you have set up changes.
The only parameter that you need to define on this type of trigger is whether the automation will be triggered for every value change, or whether it will be triggered on the first value change only. You can choose either option from the available drop-down menu.
After you are done, click on the Save button to have your trigger ready and waiting inside your workflow.
Please keep in mind that any changes in your Email lists and/or your Custom Fields may affect your automation when you use this type of trigger, so make sure that you take this under consideration when you make such changes. To move ahead with your current design make sure that you learn how to use the Conditional / Control steps and how Actions work.