My sil does relief and says the key is being organized as sometimes the call comes late at the last minute!

Are you known already at any local schools? Proving yourself will be the best path to regular work (imo) I know we definitely have a priority list of staff and then the "we're desperate" list lol!

Are you primary or secondary? Maybe prepare a "kit" of activities in case the person you are relieving hasn't left anything etc?