Since you have specific characters in mind, get your practice materials, cheap dolls, etc, as Victoriavictrix says, and practice the techniques you think you might use and get comfortable with the process. While you're practicing, start browsing dolls to see which ones are closest to your vision of how you want to present the character.
Steven's Universe has highly stylized animation, perhaps you'd like a more realistic look? Or do they need to be like Kd's beauties, perfect matches?
Do you want them to be to proper scale, as they are in the cartoon? Or are you ok with them being the same height? As you nail down these sorts of details and select the dolls you want, the techniques you need will reveal themselves and you can start to narrow your focus.
Alexandrite or Opal might be a good idea to start with Wydonna Spyder, she's got 6 arms already.
Here's mine. ^.^
DSC02327 by
me, on Flickr
Her skin is very dark, but there's bound to be a way to fix it. (not big with the modding XD)
Hope that helps.