Start with the great circle route, and adjust for winds. For example, I took BA48 a couple of months ago SEA-LHR. It usually heads straight NE over Canada, but the winds that day were more favorable for going E to North Dakota (ground speed of 700 knots, according to the map on the IFE) before turning north to catch the track over the Canadian Arctic/Greenland/Iceland to the UK.

But start with a globe and a piece of string.