Anything that can be done can be done safely. Probably the best solution is to spend a couple hundred bucks on a professional pole saw. I did a couple years ago and it's been great. Four aluminum pole sections that will extend up to 24 feet.
The next possibility is the use of the ladder. First you want to make sure you use a ladder that is rated for the total weight of the intended load. Most household ladders are rated to 250 pounds. Industrial ladders can go up to 350 in some cases, but most of them are at 300. Read the labels on the side of your ladder. Honestly, I'd suggest renting an industrial extension ladder that will get you to 24 feet.
The next step, again if using an extension ladder, is to extend it up to the closest point of the work you can. Before you climb, make sure the ladder is set at a 4 to 1 incline. There should be a guide sticker on the side of the ladder that looks like a reversed L. Make sure the long part of the L is close to perpendicular.
Next, drive a stake ( or two) into the ground between the ladder and the tree and tie off the lowest rung securely to the stake(s).
Now you can climb the ladder WITH NOTHING IN YOUR HANDS! Make sure you have at least three points of contact at all times. On foot two hands, two feet one hand, etc. This means you will need to use your saw one handed while you hold on to the ladder with the other. Take a length of rope with you to near the top and wrap it around the trunk then tie it back to the closest rung, making sure it is tight. Climb back down and get your saw. Tie a long enough rope to it that allows you to climb into position then pull the saw up to you. Again, don't climb with anything in your hands. And remember the belt buckle rule, keep your belt buckle (centerline of your body) between the ladder rails at all times. This will keep the weight centered and prevent you from losing your balance.
Wear a pair of safety glasses, gloves, and a hard hat while working. Don't cut directly overhead. Watch the wind and make sure the tree won't be moving around. If something unexpected happens stop, climb down, and rethink the operation.
Before you even get started, make sure you are well clear ( at least 10 feet) of power lines. And make sure someone knows what you are doing, or better yet have a ground person that can help out and stand by in case something happens.
This ought to see you through. Or hire a professional.
Good luck!