You will need an electric miter saw to do this. Borrow one or buy one used on Craigslist. Make sure the blade is sharp. Ideally, you should use a nail gun with finish nails but you can do it old school by hand.
The trickiest part is the apron on the bottom which should be "returned" for a finished look.
However, this will get you started. Keep at it. You can do it!
Thanks, that picture was alot of help.. but does the "head jamb extension" and "side jamb extension" actually get nailed into the actual Vynyl window, or the drywall ?
You only nail into the wall through the drywall. Do not nail the window. Remember, too, that you have to build the trim one board at a time. Read the entire article before buying wood. Match the wood style in your house so this window does not look like some "add on."
Oh, in case it does not say it, you have to counter sink the nail heads so you can fill the holes and paint over them. This leaves you with a nice flat finish.
Keep asking questions . . . you can do this one step at a time . . . just remember to hit the head of the nail, not the finger nail.