I want to build a good relationship with my co-wife but don't know where to start. How?

Start with a clean intention and realistic expectations; a good relationship does not mean being close, it is enough to have respect and no hostility. Respect each other's space and privacy, and avoid asking or talking about the internal affairs of each other's homes. Start with simple things, such as being courteous when you must deal with each other about the children or family events. The Prophet ﷺ said: “be servants of Allah, as brothers” (Sahih Muslim, Kitab al-Birr wa as-Silah wa al-Adab). Do not force closeness if the other side is not ready; give it time. If your husband tries to stir up trouble between the two of you, decline politely. Courteous cooperation benefits the children above all. See relationships between co-wives.