A master switch is just a regular switch that you have told the other devices to follow. So when it turns on, they turn on. When it turns off, they turn off.
The master can be a physical switch or it can be a virtual switch. Virtual just means it doesn’t exist as a physical object, it’s just software. But routines, rules, and smart lighting will treat it exactly the same as a physical switch.
If you prefer, you can just use one of the existing physical switches in the room to be the master. It’s up to you. But the reason why most people use a virtual switch as master is otherwise anytime the physical master switch changes, the whole group will change, and people usually like to be able to retain the ability to set each individual physical switch as they like.
So let’s assume you want to use a virtual switch as the master. The first thing you do is create the virtual switch. For your use case, it sounds like it’s best to just use a binary switch type, that is one that works like a regular light switch and is either on or off. When you turn it on it stays on until you turn it off. When you turn off it stays off until you turn it on again.
Once you create the virtual switch, you’re ready to tell the other lights to follow it.
There are several ways to do that. You can use the very popular rule master Smart app. Or you can just use the standard smart lighting smart app.
So now you’ve created a virtual switch to be the master switch and you set up a rule telling the other lights to follow that one.
From now on, any time you use the virtual master switch anywhere in SmartThings, whether it’s a routine, a smart lighting automation, rule master, or any other smart app, you just include the master switch and any other lights in the group will follow it. 
