Under the Mountain

First, find a highly original story such as a book by Maurice Gee.

Second, get a named actor (Sam Neill would do and he's a pretty good sport). Some aspiring young amateurs will come cheaply -- and they'd probably like an excuse to get out of school. If the young ones are meant to be sympathetic characters, so much the better if they're good looking.

Third, leave out all the originality of the story, and replace it with your favourite Hollywood formula, particularly ones with truculent teenagers. Keep most of the character names or you might be accused of originality of your own. Keeping the location might be useful, but not necessarily. Maybe not a good idea to change the location to New York because too many people know what it looks like, and it would cost too much to film there.

Fourth, get some nifty special effects. Fire always makes a good spectacle.

Fifth, get some cheap hair-dye to make some of the main characters' hair red, but don't mention what red hair has to do with the story -- maintain a bit of mystery.

And there you have it.

Easy.