Start with a 4" thick slab of ice.  Place it on a piece of carving  foam

Using a layout jig set at 12", scribe lines parallel to the edge of the block

Do this in both directions

Using a chainsaw, Cut right next to the first line

Turn the ice & cut right next to the second line. (always cut on the outside of the line)

The layout jig makes this very easy

Learn to carve this into an Octagon