I want to write a story mainly in third person, occasionally switching to first person. How would I do that most effectively?
Make sure you start a new chapter or section when you make a switch in either direction, unless it is short. Make it obvious to the reader. In some cases you might want to set aside the page with the first person and use a different font or a box or a background shade. You can also set text in a box for short ones. Or indent it like you might a song or poem you were including.
What's important is that your reader is not confused by it.