[thewanisdown]

Look at Crucial Conversations. Don't take it as gospel, but it's a good starting point.

Stick to facts. Clearly state your expectation, and show how they are not meeting that expectation. Then place the ball firmly in their court. The goal is not fixing the issue for them, but getting them to take responsibility for fixing it themselves.

Refrain from creating a "shit sandwich" by putting the critique in the middle of praise. That makes the conversation ineffective. These conversations are never fun, but they are important to have, and you eventually get used to it.