Now - type something, move right (or left) hand to the mouse - highlight - move mouse to menu - select - press mouse button - find "home" row again and start typing.
While that's certainly one way to do it, I've never seen anyone actually do it that pessimally.
'Cmd-B' to start the bolding (or 'Cmd-I' for italics), type whatever you need, and then 'Cmd-B' again to end the bolding. Hands never leave the keyboard. Or if you need to italicize something that's already there, select the text (keyboard is usually faster if it's just a few words away, mouse is often faster if the target text is far from the text-cursor location), and hit 'Cmd-I' (though if your hand is already on the mouse because you used it to select distant text, then it might be faster to pick from the menu).
Short version: TMTOWTDI; the "right" one might depend on the context.