I would strongly recommend completing all tutorials before attempting this so you understand the basics of takeoff, landing, trim, and navigation.
Go to Activities > Bush Trips > Breckenridge to Mariposa Yosemite. In this bush trip you'll be flying the Savage Cub, a small plane that can be a bit finicky and is easy to fly too fast resulting in an overstress crash. I found I had to pretty consistently trim up (aim the nose down) about halfway from center to the edge of the green (in exterior camera) to keep the plane from wanting to climb constantly. Try to keep your speeds around 90 knots, get your trim situated so it flies level, and don't make sudden movements and you should be good. Also when landing, "pump" the brakes, meaning don't just hold down or you could end up flipping the plane forward on its nose. As long as you feather it, couple seconds on, 1 second off, etc., you can avoid this embarrassing mistake that will cause you to have to redo the whole leg.
This bush trip consists of 25 legs and will take you a little over 9 and a half hours in total to complete. I haven't found any way to refuel the plane other than going to Options > Controls > Keyboard, setting Filter to All, and searching by name for "refuel" and binding the "Repair and Refuel" option to a key (I used the backslash key). Every few legs you will need to use this button to refuel your plane when it gets low.
The only thing you can't do that will void the achievement is use the "Back on Track" feature which shows you where you are on the VFR Map to help you if you get lost. (Commenters have noted that sim rate changes and dev teleports also may void the achievement). Other than that, if you just follow the descriptions in the Navigation pane, it's pretty easy to follow the landmarks it gives you and get where you need to go. However, I did find it useful to have Google Maps open on another monitor or device to give a little extra help in identifying where I was. It can help you make sure you're on the right road or following the right river, etc. If you do get lost to the point where you just can't find your way, you can always pause and restart the leg. Just make sure you do not use the Back on Track feature. You can use the smart cam feature to help you find the airport or airstrip you are supposed to land on when you get close to it ( by default if you're using a controller, s by default on the keyboard).
Enjoy the beautiful scenery on your way to getting what is likely to be a pretty nice ratio achievement for a while to come.