If I were lost in the North American woods, how could I make a signal fire that would attract attention many miles away ?
I found this image. I think this is good method since you can prep several signal structures depending where in your camp you are, and where the help might be approaching from. The bigger it is the more smoke it will produce. This way you just have to keep small fire going and, when you need to, you will light up the structures.
Sometimes it is not the size but the contrast that will make you stand out like sore thumb. If you are stranded in winter and everything is covert with snow you are better off with black smoke rather than white. Although I don't know what plants could produce black smoke. I know tires and oiled soaked rags do.
Also you could do tree torch but I don't recommend it.
Build a tripod with 3 branches, then make a platform with 3 more about 1/3 of the way up the tripod. Prepare a tinder bundle and kindling as you would for a normal fire (dry grass, small bark flakes and moss for the tinder and small, dry branches for the kindling.)
Once you've got the basic fire going, green, leafy branches and damp moss can be used above the fire to produce a lot of smoke when the flames hit them which should act as a good signal.
If you've alerted search & rescue and you're waiting for air support, prepare the fire so it's ready to go but don't light it right away, wait until you can hear them coming, especially if you're in a situation where you don't have much fuel.