Firstly, download a pattern from online and put it in your Desktop.
Make sure these are .PAT files.

Now, create a new document in photoshop, and create a new layer.


Then right click it and select 'Blending Options' which should open up a new window.

(Or just double click the layer, in the layers window to bring it up automatically)

Now tick the box 'Pattern Overlay' and the default pattern will apply.


Next to the preview of the default pattern, click the drop down arrow.


A selection window will open with more default patterns, click the settings icon.


This window should open up, Select the option 'Load Patterns'.


The upload window will open, selected your previously downloaded pattern and your patterns will be added to the list.



Now click your newly added pattern and it will apply to your layer.