Step 1: Frame
Important: if you’re a kid, do not attempt any of this without one of those “grown-ups” helping.
So: the frame is constructed from four planks that have been glued together to make a rectangle. You’ll need a miter saw to do this - it’s a tool that lets you make precise lateral cuts. There are two types of cuts: 45 degrees or straight.
Straight cuts are much easier overall, but tend to leave noticeable seams on the final product where the edge meets the face. On the other hand, a lot can go wrong with angle cuts (usually because the angles end up not being exactly 45 degrees and then they don’t connect with each other properly), and they’re much harder to do.