Flexibility in metamour relationships, as in all things poly, is your best approach. In practical terms, this means seeing your partner's other partner as a person, not a projection of your own fears and hopes. The best approach is the same you might take with a friend of a friend: be open and welcoming, look for shared interests, ask questions. Take the time to get to know them, but without being pushy or intrusive. Make a warm and welcoming space for them, but don't try to force them into it.