When someone isn't performing to the best of their abilities, you need to have the difficult conversation about how they can improve. Giving negative feedback is as hard for you as it is for them, because nobody wants to be cast as the villain in the game of life. There's an art to doing it tactfully which reduces the risk of hard feelings and a bad atmosphere.
First off, make sure your constructive criticism is just that – constructive. Negative feedback should always be given with good intentions, because that's the only way people learn from their mistakes. Offer them some positives around how you know they can improve because they have potential, or it's just one aspect of their job where they're falling short.
At the same time, you don't want to sugar-coat what will be a bitter pill. If it needs to be said, it needs to be said. When someone has to improve, they need to be told – it just has to be done in a way that means they will learn from it and not shut down.
Finally, you must be specific. Rather than bombard them with a whole list of issues, focus on the most pressing concern. The rest can be tackled one at a time further down the line. Also avoid generalising or bringing others in – you don't want them to think other people have been gossiping and bringing their shortcomings to your attention.
You need to give criticism which is going to strengthen the recipient, not demoralise or anger them. Show tact, compassion and good intentions – that way, everyone's a winner.
Make sure your criticism is constructive
Offer them some positives
Be straightforward
Be specific
Don't bring other people into it