The first step is admitting the possibility

Rob Silver lays out the conditions for a coalition.

Do not rule out a coalition prior to or during an election campaign - if you rule it out, you deligitimize it as an option regardless of "what is allowed" under our system of government. In other words, if it is something you are considering, be honest and clear with the voters. It is ultimately their country and they get to choose the government they want.