Substitution rules vary. Some recreational leagues use flexible rotations so everyone plays; others use a more formal format with limits, lineups, and specific entry positions. The organizer's rules always control.
Ask before the first match whether substitutions are unlimited, limited per set, or handled through a rotation pattern. Learn who signals a change and whether a player must return for the same teammate. In organized play, substitutions normally happen during a stoppage with the incoming player ready at the designated area.
Captains should know the starting order, and players should communicate early when they need a break. In recreational leagues, agree on a simple fair pattern so nobody sits unexpectedly. If a change is unclear, ask the official or organizer rather than guessing mid-match.