As we begin to explore right angles, we first discuss the degrees involved. From our prior knowledge regarding quadrilaterals, we know they are made up of 360 degrees. We further the discussion by looking at a quadrilateral that is made up of four right angles. If there are four, and they're all equal, they must measure 90 degrees.
After this discussion, we decide how we might find right angles within a shape. This leads us to using the edge of an index card. I find this helps those kiddos who struggle a bit more with spatial concepts.
After lots of playing around with shapes and labeling their angles, we move on to combining polygons to make right angles. This is a bit more challenging. This always leads to great discussion about what size angles might we pick when trying to make a right angles. The kids can see they need to be looking for angles smaller than 90 degrees, otherwise their angles are going to be too large. As the kids found angle combinations that worked, they recorded them in their journals.