“Star” actually refers to two different but extremely similar formats. For both exactly five players are required and it is somewhat of a team variant. The players immediately to each player’s left and right are their allies and the other two players are their opponents. You can attack anyone but spells which affect opponents do not affect the players immediately to your left and right, as they are not opponents by the rules of this format. The format is like the color pie, shown on the back of all Magic cards, or this image here:
White’s allied colors are Green and Blue, right next to them, and its enemy colors are Black and Red, which are not right next to them. That’s the basis of this format.
After that, there are two aspects which you can only either way on. To really get the flavor of the five colors battling, some players prefer each player build a mono-color deck for the format.
I admit, I haven’t done this, although it sounds fun if you already have enough mono-colored decks among yourself and your friends to go around, or have enough resources to build one. Personally, I like easily being able to add formats to what I’m already doing. Getting Archenemy schemes, Planechase planes, and building a Chaos stack might not be that easy, but they’re easy to add to games with decks that are already built. Especially if you want to play Star Commander, that requires building a new 100 card deck, and I really wouldn’t enjoy a mono-colored Commander deck.
As I said, having the mono-color deck option sounds fun, but it’s just as easy to ignore that part. You still have the different allies/opponents set-up than normal that can put a good twist on playing a game of Magic. Just as I know some people in my old playgroup didn’t like 4 player Commander, I also know some people who don’t like 5 player multiplayer because the more players mean more board states to have to worry about and a greater likelihood of board stalls. Star can help with both of those problems for 5 player groups if you’re experience that. Even if you’re not, it’s never wrong to try and renew the game a little.
The other part of the variant that depends on what you decide is the turn order. I’ve read that the turn order stays the same and I’ve read that you alternate every other player (i.e., White, Black, Green, Blue, Red in the above diagram; this is done to balance the game, as otherwise allies play right after one another and could rush an enemy before their turn happens).
I’ve even read where the White player always goes first instead of choosing randomly if you’re playing with the mono-color decks. I’ve only ever played the normal turn order. It didn’t seem like we had issues, but we were all pretty casual players, so maybe that affected things. I would try the normal turn order first, and if you find you’re having issues the switch it up.
Although the Star format is only run with exactly 5 players, you can have a similar experience with any number of players greater than 3: Simply play with Range of Influence 1 and Attack Left and Attack Right Only options. Range of Influence 1 means you can only target the players immediately next to you, and that spells and abilities you control only affect those players immediately next to you. The Attack Left and Attack Right Only options would mean you could only attack the players immediately to your left and right. Using these options work for playing games with 6 and more players to limit game decisions, and would work similarly to change up the ability to attack and target everyone in the same way that running the Star format would.
Whichever way you play Star, or the similar variant I just mentioned, I suggest you randomly seat everyone. Otherwise, players selecting their own allies and enemies doesn’t always work for enjoying the best a variant has to offer.