What do you use to pipe scroll work onto fondant? Buttercream or royal icing? or Does it not matter? Is one better than the other?

Also, do you just pipe on random swirls or do you need a pattern or something?

I have only used buttercream for piping as royal icing gets much harder. Also I personally have only used a press to make indention's where I would pipe scrolls because I haven't mastered that technique. But many people do it free hand very well. A turn table is key in the free hand piping for sure.

Buttercream will get blurry edges as the fat will start to soak into the fondant. Use royal if you want crisp piping. Scroll takes a lot of practice, if you aren't experienced in doing it then I suggest you use a pattern.