Here is how to assign tags to to-dos and projects:
When creating a new to-do or project, or when editing them, click into the Tags field, then enter different tags via the keyboard. Whenever you completed entering a tag, press the return key.
Open the Tags window by choosing Window > Tags. Then. drag and drop tags from the Tags window onto the to-do or project that you want to tag.
If you have specified a keyboard shortcut for a tags, you can select to-dos or projects in a list and simply press that shortcut. This will either add or remove the tag, depending on whether it was already assigned to that item before or not.