Welcome to TST. It would really help to see your swing. You can post a video in the ‘Members Swings’ thread and get some great advice. Why your trail foot is lifting is most likely a result of some other mechanic you’re doing and that needs to be diagnosed before doing any drills.
There are many YouTube videos that show ‘early extension’ drills. Maybe you could watch some and see if one represents what you’re doing wrong. Cheers.