Cut a wooden square. The dimensions equal the length of the nails. This is approximately 2 1/2 inches, but may vary a little. From experience, it would be better if the block were oversize by 1/32 to 1/16 of an inch on each side.
I used 1/2 inch angle iron to make my jig. The piece shown (red paint on the end) is a piece of scrap for purposes of this photo. It is a little short. Weld the jig where you see the red paint. Check to be certain heat from the weld has not pulled the corner so it is no longer square.
Weld all four pieces of angle iron so they fit closely around the wooden block. My finished jig is at the right of the photo.
I debated with myself about the bother of making jigs. But, as friends saw the finished Jerusalem cross I began to get requests for more and more copies. A good set of jigs makes it easy to produce consistent duplicates.