Click on 'File' at the menu bar and then click on 'Save As.' Name the document. Then, just below the file name is the dialogue, 'Save as type:'. Choose 'Document Template (*.dot).' You'll notice the directory at the top of the dialogue box changes to 'Templates.' Click 'Save' and you're done.
Why create one? _ "Re_inventing the wheel", is no fun. If you work for a single organisation it's likely that your templates could last a long, long time with no need to go back to the design board every time you are asked to create a new manual or user guide. When you work for yourself it's a great way to show clients that you have the skills they are looking for without breaking any confidentiality clauses with your existing clients, and with just a little bit of luck they'll adopt your method of working so you can continue to recycle your past efforts too.