To create shortcuts for your tags:

Choose Window > Tags.

Select a tag and double-click on the right side of the row.

Enter the one-letter keyboard shortcut and press return.
You can create shortcuts for your favorite tags. You can later use these shortcuts to quickly assign tags to to-dos.
Choose Window > Tags.
Select a tag and double-click on the right side of the row.
Enter the one-letter keyboard shortcut and press return.