A healthy social life consists of having people at various degrees of familiarity, who can move up and down closeness spectrum.

You start with setting specific relationships. Your gym buddies, your work buddies, that guy who's always at the bar you go to. For most people, their longest lived friendships started as school friends, or their parents' friends' kids. IMO, most people are able to figure this type of socializing out.

The next step up in familiarity involves building a relationship that is not fixed to a particular setting. You take a setting-specific friend, and invite them to something else. The easy way is the sort of thing where the place/timing is adjacent. You go, "Hey, gym-buddy, want to grab dinner after our workout? I've heard about this new Ethopian-Italian fusion place and I've been meaning to go". Once you reach that level of repoire, you start inviting them to things that are not immediately adjacent, like "yo, ACDC is going to be in town on the 20th, want to see 'em?". IMO, this is the step people fail at. They never make the step of inviting people to shit, and then end up with a bunch of work friends that they never see again after job hopping.

Then you make sure to see them in social settings at least every other month or so, and bam, friends. Then you start introducing your friends to each other, so that you end up nucleating a friend group. That makes it much easier to plan things, because you don't need to say "when is everyone free", you just make plans and some subset of people show up. Ideally you keep this up by establishing some sort of regular gathering, where inviting your other friends is encouraged. This lets you meet your friends' friends, which will expand your social life further. Sometimes multiple friend groups will merge, which again adds a bunch of people to your 1 degree and 2 degrees of removal set.

Beyond that basic process, the only key tips are 1) alcohol helps, there's a reason it's been the social lubricant of choice since the foundation of civilization. 2) It's okay that not everybody you ever interact with ends up a close friend. Knowing people helps you get to know more people. 3) Relationships are built on mutual utility. Most friendships exist because the people involved find spending time with the others more enjoyable than spending time alone. You can tip the scales in your favor on that by being enjoyable to spend time with. You tip the scales even further by being likely to say "yes" when invited to stuff. People don't like the feeling of being rejected, so someone who will routinely be down to hang when invited will get invited to hang a lot more than someone who is more picky. Don't agree to stuff where you'll be miserable, but if the options are a quiet night at home and a mediocre night of bowling with your friends, go with the bowling.

These tips are offered often and are presumably working for someone. As someone who has followed or tried to follow them for most of my life, there are lots of ways these tips might fail to serve:

  - Places you visit often might be fundamentally antisocial: e.g., a gym where everyone wears headphones.
  - Tickets to popular events (AC/DC) are astronomically expensive and sell out long in advance.
  - Inviting people who you don't know well to smaller, local, less spectacular events tends to bore people.
  - If someone is your first local prospective friend, inviting them and only them out constantly can stress an already tenuous relationship.
  - Introducing people to each other may reveal that they don't enjoy each other's company, so now they're both less interested in hanging out with your group.
  - Your prospective friends might have no local friends of their own, so they depend on you disproportionately for all of their socializing.
  - Your prospective friends might not drink alcohol or enjoy hanging out in bars, so hanging out at a bar no longer feels very inclusive.
  - Some people have a tight budget and don't deal well with the perceived obligation to spend money when out with friends.