make sure the ruler you are using is 1/2" thick...the 1/4" or less rulers we use to cut our fabrics are not good for long-arm work- the hopping foot can jump right up on them & break needles, throw machine out of time, cause all kinds of problems- most (long-arm) companies carry rulers specifically for 'ruler work' with your machine.
you hold the ruler- and guide the machine foot along the side of it.
an (extension table) is needed for your machine-to have a 'base' to give your ruler a stable surface.