Look at each half and decide which face is going to be on the inside. Mark the centerline on the pieces and position the axe handle over them. Use a soft pencil to outline the axe handle on each interior face.
Use the rasp to create a shallow rounded pocket for the axe handle to lay in. Work from the middle out to the edges just up to the pencil mark.
Lay the handle in the pocket and sight under it looking for high spots in the pocket. What you want is a divot that positions the centerline of the axe handle on the interior surface of the hammer head. The top portion of the pocket can be slightly wider since you’ll be driving a wooden wedge into the handle to spread it out
Next up: Final glue-up