You can associate custom letter templates to a return print set by assigning a default print set and return configuration set to an organizational unit (office, business unit, or region). 

1. Open Dashboard and click Configuration on the navigation panel.
2. Click Settings and defaults under Firm.
3. Click Tax > Return Defaults on the navigation panel.
4. Select the Print Set tab.
5. Select a year from the list of licensed and available tax years.
6. Select an organizational unit from the grid, and then choose a print set from the Print Set list.
7. Click OK to save your changes.
8. Assign the client to the selected organizational unit on the General tab of the client profile.