Listening and readback
You hear a callsign, a turn, a heading, and a flight level. You type them back. The checker is tolerant of wording and strict about the numbers. The real listening tests are not published here.
A reliable readback order
Callsign, then heading, then flight level. Play the message once. If you miss a digit, leave the miss. Replaying until you are perfect trains memory of that sentence, not the first pass. Headphones help. The room matters more than another attempt.
Similar callsigns are a paid variant on purpose. 421 and 422 are where people swap a heading into the callsign. Write the callsign before you write the numbers that follow it.