If you are released from your contract by the club, you’ll probably want to find your way back into management as soon as you can. The same Job Security screen lists all available jobs which you may apply for by selecting the club and then clicking ‘Apply for Job’ from the ‘Actions’. These jobs will also appear on your Managerial Overview screen for the duration of your unemployment.
I got that from the FM Manual. But even with the vacant jobs I can't see where it is that I apply?
If I remember correctly it is also possible to improve your chances of being appointed as manager of a team with a manager in an insecure position by telling that you are interested in managing that particular club. In the best case scenario this will speed up the board in terminating the contract of the current manager and offering the position to you. But as sayed this only works (best) if the position is insecure.