First add the red Transform (Back) frame (both frame and P/T box)
Now go to Text > Power/Toughness, and you will see that the font color is automatically set to the right color. You can also enter the other stuff at this step, name, card text, artwork, etc.
Also, don't forget to add the correct Transform Icon!
Finally, you want to go back to the Regular - Nyx (Theros) frame and add only the red Frame.
That's it, you should have the card you like now! Except, you'll notice that the P/T is reset to black. Bummer! Easy fix though! Just head back into the Transform Back frame without adding anything!
There! Now you're really done!