Choose the caption bubble to begin. If you want to upload a custom bubble or alternative image then submit the caption first, then edit the comment and attach a "Place img". Once the image is approved you can use it by editing the caption again.
I personally haven't played with it, but according to it's description it's basically a really tightly woven fabric. So based on it's description it's probably about as smooth as an allsop, if not smoother.