You've got some things to get figgered out here:
-head (shape it to have sharp edges on the front, but not where it is lashed into place)
-shaft (I think this is the most important part-good strong, light, straight, smooth piece of lumber-you have to throw it, remember?) Also, the shape of the spear shaft where you lash the head on is important to how the lashings work.
-lashings-sinew, rawhide shrink as they dry, so they are on the list.
You can lash a bayonet to a bamboo pole with paracord, and have a great weapon, but it isn't necessarily a spear. Throwing spear? Stabbing spear? What's it for?
I know these aren't really answers, but they may help to get you the info you are looking for.
Edit: obvious question-a stone (flint) head, or a bronze head? Bronze allows you to make a socket, but stone generally doesn't