Carefully hang the header board in it's chosen spot. I won't cover the basics of hanging something but all the same rules that apply to pictures, etc., apply here.
Place a hook through the top hole of each piece.
Hang the the pieces, in numerical order under their respective birth month, by hooking it to the hole in the piece, or eyelet above it. Remember to place the piece with the name of the month at the top of it's respective column.
When there is an addition to the family simply grab an extra piece, paint and label it appropriately, and hang it in its proper place.