Wondering if anyone can give advice on how to add how to add “Yes” (check mark/tick) and “No” (cross/X) marks in glyphs, how do you label it? thank you
For the symbols like this, there are not always dedicated code points defined in Unicode. You need to look though Unicode and find one that matches your use case and appearance as best as possible. You can do that in the emoji and symbol browser from macOS (from the keyboard menu item) or use UnicodeChecker or direct at unicode.org.
How to actually add glyph, is explained here: Adding glyphs to your font | Glyphs
Does this help?