Lady of the Lake has a beautiful name but is kind of a pain to make. If you're worried about your half-square triangles coming out perfect, bump measurements up to an even 3" and 9" then trim after sewing and pressing.
Two contrasting fabrics
From each fabric, cut one 8 7/8" square and 10 2 7/8" squares.
Mark a diagonal line across each of your darker squares. Make sure the 45 degree mark on your ruler is aligned with the edge of your fabric. (Ignore that my example already has a seam line!)
Pin light and dark fabrics together. Sew 1/4" on either side of the marked line. Since we're sewing so many, it's beset to chain piece in this step. To chain piece, sew 1/4" from the marked line. When you reach the edge of your fabric, raise your presser foot and pull your square slightly forward. Place another set of squares under the foot, lower, and sew. Repeat!
Press seams to one side. Trim squares (if you added extra).
Sew your squares together into strips and press seams. You will have two strips made of four units and two strips made of six units. Make sure that all of your squares are oriented identically. Check This page to see how squares should be laid out-- I messed up.
Attach strips to your center block. Press seams.