The number of lands you control is checked as this scheme’s triggered ability resolves. You can’t play a land during your turn before it resolves.

The opposing team chooses one creature controlled by each player on that team to sacrifice, then the chosen creatures are sacrificed simultaneously. If you control six or more lands, repeat this process.

If a creature has an ability that triggers when it or another creature dies, that ability won’t resolve until after the scheme’s triggered ability has completely resolved.

If a creature has an ability that triggers when another creature dies, it will see other creatures sacrificed at the same time as it. If it’s sacrificed as one of the first creatures, it won’t see the creatures sacrificed the second time.

In a Supervillain Rumble game, first the next player in turn order chooses a creature, then each other opponent in turn order does the same. Then the chosen creatures are sacrificed simultaneously. Then repeat this process if you control six or more lands.