Once you have created your custom fields, you'll need to attach them to the service you want them to apply for. To do that, go to Setup->Services:
Then either create a new service or click on a service you've already created from the list available that you want to apply the custom field to:
In this tutorial, I just clicked into the service "Hour appointment". Once in the service screen, scroll about halfway down to the option to "Form for Custom Fields: Select a Custom Field Form" and choose from the drop down which set of custom fields you want to apply:
Once you've selected the fields, scroll to the top of the page and go to the "Staff" tab:
Once you're in the Staff tab, there is a button to Save any of the changes you may have made while editing or adding a service:
Again, if you need to learn how to create custom fields, please see the documentation for it here.