That's all you need. Go up to a person and say Hi. Then express genuine interest in what they have to say, or at least fake it well enough.
Then don't make the mistake I usually make, don't stop being interested. Everything a deep and long-lasting networking connection is, is the fact you actively kept in touch.
Yes, we're engineers, no this isn't easy. But you just have to do it.
/me should really start making a habit of keeping in touch