Before just telling your significant other you want to be in a polyamorous relationship, research what it means. There are several different styles and ways that people follow through on this tool. You’ll need to research and discover what would work best for you.
If you have an existing relationship that you don’t want to destroy, then have a conversation with the person before doing anything else. Explain that you want an open relationship and what that means. For some people, that means just being sexually active with others, and for other people that means loving others and building relationships simultaneously. Some partners may prefer a “don’t ask don’t tell” kind of relationship and others may want to get to know all your partners. The key is that everyone must be on the same page with the kind of polyamory they are agreeing to.
Types of nonmonogamy include:
Swinging: when you and your partner are committed to each other but have multiple sexual partners at the same time.
Open relationships: when you are committed but are also okay with the other having sex should they find someone they are attracted to.
Metamours: when all partners live within the same household. Here there are two kinds. One is the Kitchen-Table Polyamory, where everyone gets along well enough they can sit at the kitchen table and drink coffee or have a meal. The other is the Parallel Polly is when the other partners cannot be in the same room with each other.