Checkout my strumming course: Strumming 101
This pattern forms the foundation for most rock punk and some pop music. It is a really easy Paran but you can easily add some really hectic variations so take your time to master the basic pattern.
Basically just add up strums between the downs anywhere, but the most popular variation that is quite hard for beginners is:
https://www.loom.com/share/482b7c41a6254e65b11f7e8d55972e25?sid=fe073570-f979-430f-8894-38b6e4955512
Practicing with these grooves is the BEST thing you can do to improve your strumming.
Slow
D d d D d d D d Straight 85bpm.m4a
Mid
D d d D d d D d Straight 100bpm.m4a
D d d D d d D d Straight 115bpm.m4a
Fast