What works for me is engaging in group activities that interest me (in my case that's mostly community theatre), and spending time talking with other adults engaging in that same activity, and from time to time inviting them to hang out together away from that activity. (The latter is useful for identifying people who aren't actually interested in being friends with me, just in the group activity.)

At the very least, they share an interest with me, which is a plausible place for a friendship to start.