Prior to associating a custom field to an asset, you must first have created a custom field.
To associate a custom field to many assets:
Navigate to Assets in the left side menu;
Use the filter, or the advanced filter (located to the right of the filter field) to isolate the list to only those assets that are to be associated with the custom field (nb. a filter must be used before checkboxes will appear to the left of the assets - see image below);
Check the box to the left of the asset, or check the box at the top of the list view to select all assets contained in the current filter (nb. 'all' may go beyond simply what is currently visible on-screen);
Click the Attach custom fields button;
Nominate which of the available custom fields (ie. you must already have created the custom field) are to be associated with this specific asset or material;
Optionally, add a default value that will be added to all of the selected assets;
Click Confirm.
To associate a custom field to one specific asset:
Navigate to Assets in the left side menu;
Using the filter, or the advanced filter (located to the right of the filter field) if necessary, locate the specific asset;
Click the name of the asset;
Click the Custom Fields tab;
Click the Attach custom field button;
Nominate which of the available custom fields (ie. you must already have created the custom field) are to be associated with this specific asset;
Click Save.