To smith a adamant hasta you must use a yew log, an adamantite bar and a hammer. There is also a requirement of 75 smithing. Simply use the bar on the anvil outside Otto's hut or in the Barbarian Village to make an adamant hasta, as well as gain 125 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.