First, download Anki and complete the installation process. You can then synchronise your flashcards with your mobile phone.

(Images adapted from https://apps.ankiweb.net)
Basically, you can create a stack of (digital) cards here. You write a question on one side and the answer to this question on the reverse. Learning happens when you try to recall the answer on the back of the card. When recalling, Anki’s powerful algorithm always gives you the question that you didn’t answer correctly last time.
It is not difficult to find suitable instructions on the Internet for getting started with Anki.