A. Finding the Actions
1. Click on the + button, located at every intersection on your workflow.
A pop-up window appears.
2. Choose the Actions tab, on the pop-up that appears.
3. Click on the gray dot to navigate between pages of available actions.
4. Click on the Then set custom field value option.
B. Setting up the Action
1. Select which custom field you want to set to a new value.
Notice that beside each available custom field, the name of the list where it belongs is indicated in brackets. If a subscriber triggers your automation, even though they are not subscribed on that particular mailing list to which the custom field belongs, then they will be added on that list automatically.
2. Set the value to which you want the custom field to change.
3. Click on the Save button.
Your Action step will be ready and waiting inside your workflow.
If the currently available Custom Fields aren't enough you can always conjure up your own Custom Fields and then add them to your Action as you see fit. Also, make sure that you explore the highly useful Conditional / Control steps which will help you design the perfect automation for your needs.