If you have to write an invitation letter for an overseas visitor, you can find some very good advice in another answer on this site - please click here.
As you can see from this example, the main thing is to remember to include all the necessary information, as there can be delays if you forget anything. So you should go through the checklist given and make sure you've given all the required information. This includes proof that you have accommodation for your visitor, can be financially responsible if necessary, and that all your own papers are in order - passport, student ID if appropriate, etc.
You can find further information if you put 'invitation letter uk visa' in a search engine. This will take you to various links including actual templates for an invitation letter which you can adapt to your own purpose. You can find examples here.