IMPORTING FILL PATTERNS

First go to Manage>Additional Settings>Fill Patterns:

Then choose whether the pattern will be available for Drafting (for when things are cut), or Model (for surfaces).

Hit the New button.

Choose Custom, hit Import... button and go to folder: . . .Revit\Patterns\

Select relevant folder and pattern file.

Although you won't see them from within Revit, some folders have PDF and an image file showing what the fill patterns look like.
Use Windows Explorer to browse through these files.
Then choose one of the patterns.

You may need to apply an Import Scale to get the hatch the size you want.

In the example in the image above 1X1.5 represents 100 x 150mm, so to create a 600 x 900 pattern use an import scale of 6.

You can import the same fill pattern more than once by giving it a unique name and a different import scale.

If you are creating a Drafting pattern you can also set how the hatch will rotate.
Generally best results occur when you use Align with element.