suburbanfarmer offers sound advice in post #2. Clean up the standing and fallen smaller stuff around the base to allow a clear work area and good escape routes. This includes bucking the larger, almost horizontal downed trunk. Then assess the stump situation as the desired pull will be toward the butt and away from the hung-up crown. I am not there, but it appears that the tension side of the leaner is the lower side of the trunk. On that premise notch the top side of the trunk and begin the back cut from the underside, watching the kerf closely. If the kerf stars to close, go back to the notch and cut vertically downward. I have a piece of old oil tank sheet metal approx 1/8" x 2' x 4' and would place it where the new butt is likely to fall. The metal would hopefully prevent the new butt from "digging in" and would make dragging or winching easier. Expect the crown of the leaner to hang up on the branches of the tree it is entangled in and expect some of the limbs to break and fall. You may decide to notch and cut the trunk again, but know that the closer the leaner gets to vertical, the more dangerous it becomes.
Re the video in post #8. The sawyer appears to be very experienced and often pulls the saw from a closing kerf on the compression side just before the saw would be trapped. When things go awry he is spry enough to execute plan C and reverse direction to avoid the widow maker. If you were that experienced, confident, and daring, you would probably not have made the inquiry in post #1.