A miter joint is easy to make in SketchUp. As always there are several ways to end up at the same destination in SketchUp. I will show you one method in the video.
I like to draw my object first and then make the cut. That’s the workflow you would use as a woodworker so that’s the workflow I commonly take in SketchUp. So, I draw a rectangle with the appropriate dimensions of the wood that will be used. Then use the push/pull tool to take the 2D drawing and make it 3D. On its top face I can draw a 45 degree line that represents the miter cut. Then use the push pull tool to cut away the material.
After you have one cut done you can repeat the process for the other side of the cut. Or you can simply copy and rotate your piece into the correct orientation to complete the joint.