First you need to scan them (to do that you must have the driver installed so your scanner works) and then you need to use scanning software, either from EPSON or the Windows Fax and Scan. This takes a "picture" of your letter.
Then you need OCR software which may come included with your scanner but if not then you may wish to buy some. The optical Character Recognition software converts the "picture" to "text" as best it can, which Word can then understand. Converting typed text is best; hand-written cursive is not as likely to convert well.