Open your Google Slides presentation.

Click on a text box or insert one by selecting "Insert" > "Text box".

In the toolbar, click on the font dropdown (default is usually "Arial").

Scroll to the top and select "More fonts".

In the pop-up window, browse or search for the font you want.

Click on a font to add it to your list, then click "OK".

The newly added font will now be available in the font dropdown for use.