To smith a mithril hasta you must use a maple log, a mithril bar and a hammer. There is also a requirement of 55 smithing. Simply use the bar on the anvil outside Otto's hut or in the Barbarian Village to make a mithril hasta, as well as gain 100 experience. You will not be able to make a hasta until you demonstrate to Otto that you can make a spear.
Hastae can also be poisoned.