This is probably a really simple question, but how do you and what tip do you use to get the disc-like effect on a border. It's like there's a disc or something sticking in between each shell. I'll attach a pic so maybe it'll be more clear then my description. Any help is appreciated. Thanks!

I use that all the time! It's just the rose tip, 104. Just put it down between the two shells, squeeze & pull the tip down. Then stop pressure. I'ts hard to remember without me doing it, but I think the wide end is at the top. But just play with it to see. It's very easy & I also like to do up to 3 different flutes, staggering the colors.