Drill or punch the hole in the flatbar.

Weld the two pieces of flatbar together.

This needs to be a really strong weld! The hammer will be under a lot of stress, and if it's not a solid weld, it could fly off! (This is why the guard is there.)

Tack the shaft into the hole so there's about a 1/4" distance between the end of the shaft and the opposite side of the hammer.