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 app's settings set the type of translation you need - Text to Text or Speech to Speech.
When all is set - tap one of the flags buttons at the bottom of the screen.
If you are using Text to Text translation - the keyboard will appear.
Type a word or phrase you want to translate and tap Return button on the keyboard to finish the translation.
If you are using 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.