The initial community of HN was the readers of the blog posts^W^W articles of pg.

The initial community of Stack Overflow was the readers of the blog posts of spolsky and codinghorror (the two founders).

So the recipe appears to be to make a interesting and unique blog.

And after you have a nice technical community, you need moderation, a lot of moderation. There was a lot of work initially by pg and now by dang to moderate the site. A lot of that is invisible, but you can take a look at the recent comments by dang https://news.ycombinator.com/posts?id=dang