challenge level

Love this game and got through the story mode; however, not sure what it takes to get a gold star? It looks like all locations have 6 levels, have gotten 3 gold stars on the first location, but not sure how. Any suggestions?

I just started challenge mode and I also can't figure out what it takes to get a gold star. I hope someone can come along and enlighten us.

I'm super impressed you guys got to challenge level. I may never get there......