1 - buy 1 long blade, the one i got was 24 and is perfect.
2 - remove both old blades. they are on these rotating studs shaped like a star. so you gotta lift them straight off.
3 - put your new blade on, and this will go on the passenger side wiper stud. to make it reach across the whole window when you put it on, it needs to start about half way up the window.
thats it. to figure out the spot is just trial and error. so when you push it on, turn on the wipers and see where it covers. if you need more or less, take it off and rotate it in the right direction.
To assist you in determing how much travel the new wiper will have, suggest you wet down the windshield first. Wiper will travel farther when wet and you can then see if it´s going to have clearance issues with the A-pillar in the real-world.