Ditto what Dunster said. You need to sew on two opposite sides and then press these triangles to the outside before sewing the other two sides on. That's OK though, as a beginner one needs to learn to make friends with the "froggy" also known as the seam ripper.
In this case, I would only rip back as far as you need to to pull the triangles out and press. Then resew those areas to secure the seams.