I agree with DD, I think piping gel is kind of gross. If your covering the cake in BC you can set some aside to tint darker/different colors. You'll need piping tips and a bag/parchment bag. The fact that your asking what to use seems like you don't have a lot of piping experience. I'd definitely do some practice scrolls, lines, etc. you can print out templates and put parchment over so you can practice.
If you have any experience with fondant you can again print out or hand draw a template, tape some parchment down over that, then use an extruder (or hand roll sausages thinly) to form your shapes. Let them dry and attach to the cake. I guess they would dry flat now that I'm thinking about it so you could form them around a cake dummy to dry.
These are fairly simple designs though, you can mix it up a bit so I'd practice piping