First, admit that it exists and commit to taking positive action.
You may wish to read our article on the Silent Underperformer.
The key steps to managing poor performance are:
1) Setting clear standards and timescales
2) Identifying which barriers are in the way and agreeing reasonable support
3) Regularly reviewing performance openly and honestly
4) Being clear and transparent about the consequences of poor performance
5) Identifying when it is time to deal with the issue formally
If your business has a separate Capability or Managing Performance procedure, you should follow this and above all be consistent in its application across all your staff.