A good ladder with someone holding it firmly at the bottom. DO NOT do this alone. Make sure your base person understands not to move until you're back down near the third rung.
Don't tie off to the gutter. If the ladder falls it'll most likely just be more debris to fall on top of you. You're better off throwing a rope from the window that is properly anchored to a stud with a decent eye bolt/lag. On the ladder end, make sure you have some type of quick release system, a carabiner to an over hand loop knot or something. Make sure the rope doesn't have much slack in it that the ladder can just fall or swing. This seems kind of extreme, but if you feel like you have to anchor make sure it's to something extremely secured. Something like this:
http://www.lowes.com/pd_21795-1277-MP2150BC___?productId=3151703&pl=1&Ntt=eye+lag+screwStay on the ladder. That's a pretty good pitch and working without any fall gear on the pitch facing down it towards the gutter would be really hazardous. Don't reach further than you're comfortable. When you can't get something out of reach, go back down and adjust the ladder to the new spot.
Make sure you and the base person can hear one another well. Make sure the base person has an adequately charged cell phone readily available should something go wrong.
If you should fall, don't stay with the ladder. This won't happen if your base person is doing their job. The ladder and gravity are most likely going to do more damage than just gravity.
Base person: feet on ladder's feet. make sure ladder feet have jagged edge out. Always have both hands on ladder stabilizing it at all times. Make sure they have a brimmed hat and some safety goggles or even sunglasses for falling leaf debris. If you can, make sure they're over 150lbs.
If you're uncomfortable with heights this wouldn't be a good starter project. You'd be really surprised how inexpensive it would be to hire this out.