To use the datapack, generate a world with cheats enabled, then do /reload,
And you are done!

To enter the lush dimension just step in the portal from an ancient city. You dont need to light it.

Once you enter the lush dimension you are on a 1x1 bedrock platform. You can't build on this platform.

If you want to respawn in the Lush Dimension, do /setworldspawn so that everyone that joins your world will (re)spawn there. Or use a respawn anchor.

In the lush Dimension, it is always night. Beds will explode, but you can use respawn anchors. You can find strongholds, mineshafts & dungeons.

You can't make nether portals in the lush dimension.

That's all the info, have fun!