Prior to associating a custom field to a material, you must first have created a custom field.
To associate a custom field to many materials:
Navigate to Materials in the left side menu;
Use the filter, or the advanced filter (located to the right of the filter) if necessary, and isolate the list to only those materials that are to be associated with the custom field;
Check the box to the left of the applicable materials, or alternatively check the box at the top of the list view to select all materials 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 material;
Optionally, add a default value that will be added to all of the selected materials.
Click Confirm;
To associate a custom field with one specific material:
Navigate to Materials in the left side menu;
Using the filter, or the advanced filter (located to the right of the filter) if necessary, locate the specific material;
Click the name of the material;
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 material;
Click Save;