Lady Sif is a shield maiden of Asgard so a shield is a key part of her costume. Instructables user Amorrah used Sif’s shield from Thor: The Dark World as a reference and build the shield for her cosplay from foam board and craft foam. She started by sketching the outlines of the shield onto paper and transferring both of them (a larger outer shield and smaller inner shield) onto foam board. You can sand the edges of the foam board after you cut the shields out to smooth them. Cut the Celtic-esque details for Sif’s shield from craft foam:
Lay the design detail sketched banner paper, over a section of craft foam.
Use an exact-o knife to puncture dots on the craft foam, much like carving a pumpkin, following the design lines.
This allows the overall design is captured.
Remove now-dotted banner paper from the foam layer.
Go over the dots again with the exact-o knife, using regular slicing techniques and cut out the detailing.
Read more about the build at Instructables.