To save text as a Text Snippet:

Choose Text Snippets from the popup menu beneath the light blue source list on the left of the window, or press Command Option Shift T.

Click the + button in the bottom left corner of the window.

Drag or type the text you want to save to the text area above the dark blue-gray area near the bottom left of the window.

Type a name for the Text Snippet in the text field beneath the Description label in the dark blue-gray area near the bottom left of the window.