Adding text notes to scores

To create the text, see https://musescore.org/en/handbook/text-0. To move it below the staff, simply click and drag, or use the Offset controls in the Inspector.

See:
https://musescore.org/en/handbook/text-0
for entering text (as previously mentioned).

For generic text (as opposed to special text, like lyrics or chord names) you can use Staff text or System text. To move the text down (below the staff), you can select it and change the vertical offset in the Inspector - positive values move the text down.

Alternatively, for a more global setting you can set the vertical offset using menu item: Style -> Text -> System (or Staff). See:
https://musescore.org/en/handbook/text-style-0

Feel free to attach your score with any specific questions you may still have.
Regards.