This is a nine patch pattern which uses half square triangles in the four corners and half square rectangles in the four sides. This block uses the same shapes and positions as the Churn Dash block but uses colour placement to create a different pattern.
To draft the Grecian Square/Greek Cross start with a 3 x 3 grid
Divide the side squares in half to form rectangles.
Divide the corner squares into half square triangles
1. This is a video by Nancy's Notions
If you prefer a written tutorial Quilters Cache has you covered with this tutorial for a 12" block and example layouts.