First, nobody else near the tractor - safety is important. Put the tractor in neutral and move it to where there's some room around it.
Using a jack, lift both rear wheels 1"-2" off the ground and support the tractor on blocks or stands. Make sure tractor is in 2wd. Put transmission in low range first gear. Push the clutch pedal down so the switch allows the starter to work.
Start the tractor and run the engine at about 800 - 1000 rpm(wheels will be turning)
Then, with the clutch pedal still down and brake pedals locked together, step down hard on the brakes. The clutch should pop loose. If the engine stalls instead, try it again with engine at 1200.
Good luck.