Here is how to install a data pack when creating a singleplayer world:

Open Minecraft.
Create a new world and click on "Data Packs".
Drag the data pack into the Minecraft window. It should be a .zip file or directory.
Click "Yes" on the confirmation screen.
Move the data pack to the right hand side of the screen by clicking the triangle on its icon.
Click "Done" and continue creating your world, then click on "Create New World".
If the data pack has a correct format, it would be enabled in the world. If you have enabled cheats, you can verify by typing /datapack list enabled and find an entry named.

Here is how to install a data pack in an existing singleplayer world:

Open Minecraft.
Select the world you want to install the data pack for, click on "Edit", then "Open world folder".
Open the folder named datapack, and put the data pack into it. It should be a .zip file or directory.
Type /reload (if you have enabled cheats) or press F3 + T if you are in the world during the installation.
If the data pack has a correct format, it would be enabled in the world. If you have enabled cheats, you can verify by typing /datapack list enabled and find an entry named.
If it doesn't work then reload minecraft.

Guide of Newcomers