To start with we will be selecting an appropriate piece of wood for the hammer head. For my purposes I selected a scrap bit of 4x4 and cut it to seven inches in length. Make sure to cut both ends at a true 90 degree square. These cuts can best be done using a miter saw or table saw.
Next we need to locate where the handle will be attached. Simply use a straight edge to draw a line from corner to corner in an "X" to locate the center. Flip the wood over and do the same thing on the opposite side.
After marking your centers, run the hammer head through a router to round the edges. I opted for a flat cut for that special "Thor" look. A purist will tell you a truly round edge is better and he would probably be right.
Using a piece of dowel, broom handle, or plunger handle, cut it to a comfortable length to go all the way through the hammer head. With a table saw or jig saw, cut a notch about half the width of the hammer head.
With a drill press, run a forstner bit through at your center marks. It is important not to go through in one press but to drill from both sides to avoid splintering. The size of the forstner bit should be large enough to accommodate your handle with little to no wiggle room. It should be a very snug fit.If you want a "showroom" hammer, this is a good time to sand the hammer head before the handle is put in, my hammer is for purely functional purposes so I am skipping the gratuitous sanding.
Apply wood glue to the end of the handle and tap it through slowly with another piece of wood (since you don't have a tap hammer yet). Let the glue dry overnight and Viola! A hammer any Norse carpenter would use!