Square Shovels

Sharpen the inside of the shovel's leading edge using an angle grinder with a 120 grit flap disc. A metal grinding wheel may be needed for shaping the edge if it is in bad shape.

Sharpening the top edge enables the shovel's back to lie flat on the surface, getting all the materail into it (e.g., mulch from concrete).

General Guidelines
If the original angle is different, it is best to match that.