CMC stands for carboxymethyl cellulose. It's a fine powder that makes the sugar paste thicker/firmer (the formed parts hold the shape better) and speeds-up the drying time. Other names for CMC are tylose powder and tylo powder.

CMC is NOT the same thing as gum tragacanth. CMC is a synthetic substitute for gum tragacanth. CMC works quicker than gum tragacanth, so you don't have to wait many hours for the paste to get thicker/firmer.
I rarely use sugarpaste (fondant) with CMC to make cake toppers. I find it difficult to get a smooth finish.
I use either:
  • plain flower paste (gum paste) without CMC
  • or the 50/50 FP/SP mix with CMC
    • A 50/50 FP/SP mix is a mix of flower paste (gum paste) and sugarpaste (fondant) mixed in ratio 1:1. F.ex. 25 g of FP and 25 g of SP. You don't need to weigh the paste - just mix similar size pieces.

These are the amounts of CMC I use:
  • About 1/2 teaspoon of CMC per 100 g of 50/50 FP/SP mix if I want the paste to be quite thick (f.ex. a body that doesn't need a lot of shaping).
  • About 1/3 teaspoon of CMC per 100 g of 50/50 FP/SP mix if I want the paste to be a little softer, but still hold the shape well (f.ex. a body that needs more shaping but still needs to support a heavy head).
  • About 1/4 teaspoon of CMC per 100 g of 50/50 FP/SP mix to make parts that don't need to support any weight (f.ex. arms or tail).

If you are using different types or brands of paste you might need to use less or more CMC. So if I were using sugarpaste (fondant) instead of the 50/50 FP/SP mix, I would use more CMC (sugarpaste is softer than the mix).

Since I add the CMC shortly before forming a certain part, I don't just take 100 g of paste and add 1/2 teaspoon of CMC to it. Instead of that I take the small amount that I need f.ex. 30 g and add the amount of CMC that I think is enough (I don't measure it). I knead the paste. If it still feels too soft, I add a bit more CMC.

The more cake toppers you make, the easier it will be for you to determine how much you need to add :)