There are multiple ways to do that, and one sounds in some situtation while others in different scenario.

First one is pinch your thmub and index finger as if you're holding a pick. Then strum like that. This doesn't sounds very bright, but gives you lot of control to strum various patterns.

Second method is use your curled fingers to downstrum and thumb to upstroke. This is very bright and sounds like original pick. Caren strums like that, you can check out her tutorial as well : https://www.guitartricks.com/lesson.php?input=18320&s_id=1408

Last would be to use your thumb for both upstroke and downstroke. Few people uses it and it sounds soft, but I don't use this very often.