First you need a seperate datapack for the game! If you do not know commands, I don't recommend making games.

After you've done that, and you have an idea for what you want to make, goto the next dropdown.

In order to create your own game, we need to make it so when you turn the switch on, you can select your game by clicking text in your chat.

If you don't know how to do /tellraw, use MCStacker!

Notice that I have a run_command area make sure you fill that out with
/tag @s add <GAMETAG>You can change your gametag to whatever as long as it doesn't have spaces.
Now in our datapack we need to add the command!
Inside your tick function add this command below;
execute as @a[scores={rc=1..},tag=!inGame] run <Tellraw command that you generated>Your finished with this one!

Alright! This is what takes up so much time!
For each possible position of the player to be in, there has to be a custom model for it.
Its long and cruel but you've made it this far!
Assuming your using Blockbench,
Make a cube with these dimensions;


After you've done that,
Goto Display and goto Firstperson Left

Now, input these numbers into the boxes,




After you've done that, put the item into the game using CustomModelData.
Then use this command below for each screen in your tick function;
execute as @a if entity @s[scores={Screen=1},tag=GAMETAG_HERE] run replaceitem entity @s weapon.offhand <item you used for custommodeldata>{CustomModelData:<ID_HERE>}>Be sure to fill out the command and add 1 to the Screen scoreboard for each screen and model.

And now your done!
If The rest of what you add to your game is up to you!

If its not working for you, then you didn't do something right.
Be sure to read over the steps carefully.
If you find a bug DM @SynicalMC on Twitter with your datapack and resource pack and what you were doing.