First, there are no rules. Nobody owns the copyright on polyamory. You get to build your own to fit you and your dearloves.
One thing that comes up in every conversation about polyamory is communication. If there is any basic building block, this is probably it. If you can talk about your hopes, you're on the way to realizing them.
If you're in a relationship already, and have not talked about how you feel and what you want, and you're asking the question "How do I start doing this poly stuff?", you may have some qualms about talking to your partner. What you do will have to be determined by your own ethics and your own situation; chances are that if you ask on the newsgroup, many poly folk will suggest you talk it over with your partner, and they may point out that even if you two do not decide to live polyamorously, you may very well increase the intimacy level in your monogamous dyad by having the discussion.
On the other hand, it may all go blooey, and this is why people hesitate. On the third hand, nothing ventured, nothing gained. On the fourth hand, it might be useful to increase the intimacy level in the existing relationship and address any outstanding difficult issues there *before* having this particular discussion. Four more hands and you've got a nice statue of Kwan mYin there, and seeing as how she's the Goddess of Mercy, she might come in handy at a time like this.
Many feel that it's not a good idea to try to force a relationship into an attractive model; they favor the "relax, be open, and see what happens" approach, and quotes Pete Seeger:
"Take it easy, but take it."
If you're already in more than one relationship and haven't disclosed this yet, you will find people on the site who have experienced similar things - from all three sides - and are willing to discuss their perceptions and the actions they took.