The club calendar displays all upcoming matches. To join a friendly match, you just need to be in the lobby when the timer goes down to 00:00:00. See below for how to get into a match lobby.
It’s important to set your availability as soon as possible, so your manager can plan accordingly. You can also see which of your teammates are available. The manager can provide notes to the team, although more detailed notes are likely to be in the club forum.