Start by removing the trim from around the window, this will give you a good look at the problem. Once you have the trim removed you'll be looking at something similar to this...

![enter image description here][1]

You may notice gaps between the window frame and the wood frame or between the wood frame and the wall, these are the areas you'll want to fill to stop the draft.

Get a can spray foam (like GreatStuff they make some specifically for this purpose, which is "low pressure" so it will not bow the window frame), fill any gaps (be careful not to use too much, this stuff can expand quite a bit). Once the foam has hardened (about an hour), use a sharp knife to trim the excess foam so it's flush with the window frame. Reinstall the window trim, and your done. [1]: https://i.sstatic.net/igwSW.png

Start by removing the trim from around the window, this will give you a good look at the problem. Once you have the trim removed you'll be looking at something similar to this...

You may notice gaps between the window frame and the wood frame or between the wood frame and the wall, these are the areas you'll want to fill to stop the draft.

Get a can spray foam (like GreatStuff they make some specifically for this purpose, which is "low pressure" so it will not bow the window frame), fill any gaps (be careful not to use too much, this stuff can expand quite a bit). Once the foam has hardened (about an hour), use a sharp knife to trim the excess foam so it's flush with the window frame. Reinstall the window trim, and your done.