Don't procrastinate. If you are unhappy with someone's performance, you need to do something. Letting it fester until you have no option will only make it worse.
Make it important. Tell yourself that the help you are offering could transform their performance. This will motivate you to make the effort and your faith will almost certainly be infectious.
Be specific. Explain the gap between what they're doing and what they need to achieve. 'Your report on movie sponsorship lacked clear recommendations and also had four factual errors' beats 'your reports are sloppy'.
Make sure they have a clear understanding. Simply nodding agreement does not mean that they have. Discuss the gap with the person that you are coaching.
Do not make assumptions. Discover the reasons for the underperformance by approaching with an open mind, asking questions rather than assuming you already know what the cause is.
Set achievable goals. When people feel overwhelmed, they tend to rush around achieving little. Ease pressure by helping them decide on priorities and agree deadlines for each task, or helping them to delegate. Arrange to meet again to discuss progress for each goal.
Make them feel valued. If the reason for their poor performance is that they don't like what they are doing, help them understand how they contribute and how you value the role ('It's vital to making sure that things run smoothly' rather than 'it's boring admin'). Explain how excellent performance could lead to greater things.
Give continuous support. In between meetings, use every opportunity to praise what they're doing well and to offer constructive feedback if things begin to go awry. If you are seen to be supportive and they continue to underperform, it will be easier to take action.