First, set the languages you need for translation. You can see a detailed guide for this in here: How can I change languages in the app
After this, in the toolbar at the bottom of the main screen, set the type of translation you need: Type text-to-text or Speak speech-to-speech.
Once you have selected the type of translation, tap one of the flag buttons at the bottom of the screen.
Type
If you are using text-to-text translation, the keyboard will appear.
Type a word or phrase you want to translate and tap the Return button on the keyboard to finish the translation.
Speak
If you are using speech-to-speech translation, tap the flag to activate the microphone. Say the necessary text for translation and then tap the button once more to finish the translation.