The Spanish fourth seed, runner-up at Wimbledon last year, became the first Spanish woman to reach the Roland Garros final since Conchita Martinez in 2000.
She will face defending champion Serena Williams, hoping for a different outcome to last year at Wimbledon.
"I have learned a lot how to control my emotions inside the court and outside the court. I think it's very important, because sometimes it's not too good to show them or not controlling them," Muguruza told reporters.
"In a tournament like this you have to be very focused. It's very long. Even longer with this kind of weather that you have to wait a lot. Here I'm learning. Here just I'm putting everything into that and it's going well."
Muguruza, 22, made the most of 2010 runner-up Stosur's early jitters on Court Suzanne Lenglen and used her booming forehand to race into a 4-0 lead in the opening set.
She was also well ahead in the second set before Stosur finally made her sweat.
"I played very well until I led 5-2 in the second set and then she stepped up a gear and I became a bit nervous," Muguruza, looking to become the first Spaniard to lift the trophy since Arantxa Sanchez Vicario in 1998, said courtside.
"Emotions sometimes can be bad."
Muguruza, who was in the French Open semi-final for the first time, showed no nerves at the beginning on a chilly, misty day in the French capital.
She broke in the first game, staved off two break points in the second and then Stosur double-faulted to gift the Spaniard a second break and control of the match.
Muguruza bagged the first set with an unreturnable serve and was cruising to victory when she stole Stosur's serve in the first game of the second and held to lead 2-0.
Stosur, who lacked her usual accuracy and power in the heavy conditions, fell 5-2 behind, leaving Muguruza to serve for the match.
The Venezuela-born Muguruza was then broken to love and Stosur promptly held.
Muguruza trailed 15-30 in the 10th game, only to pluck out two aces to set up a match point, which she converted when the Australian sent a forehand wide.
Since losing her opening set of the tournament, Muguruza has won 12 sets in a row.
(Reporting by Julien Pretot; Editing by Pritha Sarkar and Ed Osmond)
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