I really enjoy using rulers.
As Three Dog mentioned, if you go through a Sew Steady/Westalee dealer, you just have to give them your model number and they will give you the right ruler foot. Their intro set actually comes with my favourite ruler of all time which is straight on one edge and an arc on the other. I use it all the time. It has very useful markings on it and comes with rubbery things so it doesn't slip.
With rulers, you have to be able to visualise that everything is 1/4" over from where it seems, as the needle is centred in the middle of the foot, a 1/4" from the edge of the ruler. Once you understand the concept, it's easy.
Some things I had to pay attention to were to not look too far ahead, because then the foot would wander off the ruler and I'd get a squiggly line and to keep my hand not just on the ruler, but also on the fabric, so I could move it all as a unit. Also, don't be afraid to use your marking pens so that you know where you're going and don't feel you have to press hard against the ruler.
The lady from Westalee has some really good videos as do many people, but a beginners class is likely great, too. (Although I managed without one so it can be done.)
Have fun!
Watson