Find two adult frogs
Feed each frog a slimeball
Wait for the frogs to enter love mode
One frog will lay frogspawn in water
Wait for the frogspawn to hatch into tadpoles
Collect the tadpoles if needed using a water bucket
Raise tadpoles in water until they grow into frogs
Keep tadpoles in their preferred temperature biome for the frog variant you want
Use more slimeballs to breed the adult frogs again