Once they've been created in the admin area, Custom Tabs are stored in an action menu to the right of the system tabs on a record view.
The order these tabs are shown in on your records can be managed from the Custom Fields and Tabs section of the Admin menu. While viewing your Custom Tab lists;
- Click 'Manage Tab Order'
- Drag and drop your Tabs into the correct order
- Click 'Save' when you're done
The order set here sets the order that the Custom Tabs are shown on the dropdown in record view.