When in Practice Mode, the game will match you only against bots (always matching by your number of trophies), allowing practice with different decks without risking trophies. So you can try different strategies and cards, mastering its different abilities.
To start practicing, go to the game's home screen and tap on the target icon (right between the inbox and tournaments icons). A message will pop out warning you that no trophies and/or chests are going to be rewarded on Practice Mode, then just click OK and start the battle with a Trainer (bot).
Feel free to practice as many times as you need to be prepared for battling with real players!