1. Log in to Portal as portal
2. Go to Navigator > Page Groups > your_page_group > Templates (*)
3. Click "Create New Template"
4. Enter Name (use only a-z and 0-9, no spaces) and Display Name (can contain special characters and spaces)
5. Check the Make available for use in this page group field
6. Click Finish
7. Click Template Properties
8. Select the Optional tab
9. Select the appropriate User Interface Template from the dropdown list
10. Click OK

(*) In Portal 10.1.4, they are called Portal Templates