Time investment, mainly. Find someone who seems cool that you know from work or college or neighborhood or wherever is convenient. Hang out with them at the convenient place to see if they would be a good friend. If so, invest time. Find or create events to invite them too, including random "just because" hangouts (but still do things, like play a videogame together or show eachother cool books or chat while taking a walk). Hanging out with someone in person outside of the convenient normal circumstances you met in is how to make an acquaintance/colleague/classmate/neighbor become a friend instead of just someone with whom you are friendly.
(Just a guess, though. I'm not a social expert.)
Asking for a friend.
(Just a guess, though. I'm not a social expert.)
(I don't have friends)