What you ask is quite straightforward. Just create the merge document. Attach the worksheet to the document. Insert the fields where you want them along with any fixed text, then save the document. See
Mailmerge with Word 2007/2010
If you keep the same filename for the worksheet each time there is no need to change anything. If not you will have to attach the required worksheet each time.
You will cause fewer problems for yourself if you have the data starting at cell A1, rather than randomly placed in the middle of the sheet.