When you generate the world the border is centered around 0.5 0.5 and set to a 50 block diameter (if the area at 0 0 is unsuitable for a spawn, you can just center the worldborder somewhere else¹ and put the spawn there, the point of it being at 0 0 is just to simplify finding your way back). Before the game is started every player has mining fatigue/resistance/weakness so that they can't mine blocks/die/deal damage.

To easily get a spawn area you can download my datapack here (it's the spawn you can see in the screenshot).
To use it, just fly above the area you want your spawn (a 15 block radius around you has to be clear of blocks, flying up about 20 blocks should do) and run the command /function sgspawnbuilder:buildspawn.
I recommend installing worldedit and worldguard to protect the iron blocks and the beacon, so no one can get the blocks or block the beacon beam.

Once your spawn is ready, move your friends (and yourself) into the spawn boxes (one player/box of course) and run the command /function sgcore:startgame.


¹If you do not know how to move the border continue reading: Firstly you'll want to make it bigger so you can move, to do this run /worldborder set <Diameter in blocks>. Once you have found an area for you spawn check your coordinates and run /worldborder center <x> <y> <z>. Now you can shrink you worldborder again by running the command /worldborder set 51 - if it is centered around your spawn you've done everything correctly.

*Note: There is an ingame timer and the exact times are all announced in the chat.

Once you start the game the border moves to a 1000 block diamater. The Blue bar at the top of your screen is a timer, when it turns yellow there are 5 minutes left to the next border, once it turns red there is 1 minute left to the next border. Overall the first timer takes 45 minutes, after it has finished the border moves to a diameter of 500 blocks over the duration of 90 seconds and the timer of the top of your screen is reset. After another 15 minutes the preparing phase ends and the border moves to a diamater of 100 blocks over the duration of 60 seconds. At this point people are so close they will probably start fighting. After another 2:30 minutes the border moves to 50 blocks over the duration of 30 seconds, and after another 1:30 minutes the game goes into Sudden Death - the border moves to a diameter of 10 blocks over 2 minutes