To Sean's question...unless you are fortunate enough to have a second sawyer carrying a smaller saw (in these parts, a smaller saw might be a Stihl 046/460 with a three foot bar or so...not smaller in most parts of the world, but be that as it may
...
If the smaller saw is at hand, then just set the kerf where appropriate, a couple of inches deep, then shut that little girl down and put the big saw in that kerf and go to town (assuming similar bar/chain gauge). Simple...both horizontal and angled face cuts (conventional or humboldt, same same).
If you have nothing but the loooong bar beast at hand, what you do is hang the bar tip in the bark, about a foot aft of the sprocket. Eyeball down the bar from there to the powerhead to see a clean
straight presentation of the bar...no wows or bows. Anything otherwise is going to result in a thrown chain.
When you have it there (some muscle generally required to hold that bastard 125+ cc powerhead in the right place
), squirt some revs and get a bit of kerf in the proper alignment. If you did it right, got your initial cut in the right place, move the bar forward a foot or two in that kerf and deepen that kerf a few more inches...all the while keeping wows or bows out of the bar presentation, then proceed per normal with the felling cuts, both horizontal and angled, using the same technique. Again, conventional or humboldt, same same.
Once you have that done, you are golden, generally...though it's still easy enough to get that long and flexible bar out of straight alignment and toss your chain even at this point.
Assuming nothing goes badly on that score, just fell the darn tree like any other
.