I have Word. Not sure what version.  Possibly Word 10. 

Here is what I would do:

1) Open MS Word.  Go to File: New.   This bring up a page to find a new template. 

2) If you see a search bar at the top, type in "envelope".  This should bring up possible envelope templates to use.  Pick one. When you have one selected, click on "create".

3) IF you want just a "plain jane" envelope, without all the frills being offered..... click your cursor on the frill stuff, then hit the "delete" key.  You will need to do this item by item.  I took an envelope template, deleted each item on there, and ended up with a "plain jane" template.

4) You should have your cursor in the top left corner.  Hit the "enter" key, to come down a couple of rows, to where you want your return address.  Hit the "tab" key once or twice to move in off the edge.  Enter your return address.

5) Hit the enter key a couple of times to move down the page. Hit the "tab" key to move your cursor across to the middle of the page. Maybe move it one-third way across.  Type in a mailing address. How does it look?  Is it spaced where you want it?   If so, go ahead and save this on your computer. I call mine "Envelope Template". You can call it what you wish.

6) Printing. First time I would print it on regular 8.5"x11" paper.  Be sure you are selected for Landscape orientation.  Print it on a piece of paper.  How does it look?  If margins look okay, save it; if margins don't look okay, change the margins and try again.  Play with it until you like the results.  Then save it.

7) Once you think it looks good on paper, I would then try it with an actual envelope.  Again, if the margins or settings or spacing doesn't look good, adjust until you are satisfied.   Again save it.

Next time you wish to print an envelope, bring up your saved envelope template, type in a new address, and you should be good to go.