You need stiff card, you can use scissors but I prefer craft knife and cutting board.
Fold card in half.
Make equal parallel cuts across the fold.
Push the cut piece inward to make a reverse fold. You could leave it like that or make further steps to pop up.
Here there is a smaller upper pop out.
Here a Christmas tree has been attached to the pop up.
The card on its side, showing the pop up tree. You can add lots of bits to this to finish your card.