Creating a pack

First create an empty resource pack, or, if you are a mod developer, you can add the folders to your mod's resources.
In the assets folder, create a folder with the name of the mod ID of the mod you are adding support for. In there, add a folder named seamless_rules. If the mod ID doesn't match, it won't work.
For example, such a folder structure could be assets/supplementaries/seamless_rules.
Don't know what the mod ID is? Look at the text before the ":" when hovering over the block with Advanced Tooltips enabled.