You will need some type of straight edge ruler since straight lines are the hardest to do on a sitdown or a long arm. I like my short one, maybe 6 inches. It might seem like a longer ruler would be easier to use, but it really isn't. You need to reposition any straight ruler frequently, and the shorter ruler doesn't get in the way as much. There is a handy versa-tool ruler that HQ sells which is nice as a kind of sampler of curves and straight. You can see some videos at the HQ site (and probably on Youtube also). I have some clamshells and some half circles, but I've only used them a few times. I find that I prefer fairly loose FMQ instead of the traditional styles so the rulers don't fit my style as much. YMMV.
Pam