What is the safe/correct way to work on gutters? From a ladder or from the roof?
Given the choice, I'd put the ladder 2-3' to the side of the downspout.  That should allow you to safely reach the downspout and a majority of the problem area without getting off the ladder.  Using a stabilizer bar so that you aren't resting on the gutter helps:



This also minimizes wear and tear on the shingles.  If you do need to walk on the roof, stay low with your feet always lower than your body, wear shoes/boots with a good grip, and if possible, have a rope attached to something fixed.  If you're going to be on the roof a lot, you may want to invest in a harness.