1) Use ACS or PMC clay that's been around a while
& re-used / re-rolled a few times. You'll notice it's not quite as smooth as the "Fresh out of the Packet" stuff you get better results that way ;)
2) Choose two texture plates (you can make them yourself or use commercial ones)
3) Spray one of them with coolslip (or oil it so it doesn't stick)
4) Roll out clay and press onto texture sheet
5) Here's the cool bit, RIP the edges leaving it irregular.
6) Let that shape dry flat & don't try to neaten it!
7) Repeat the process making top piece using the second texture. This time wet the edge of your finger & smooth the edge slightly
8) While the top piece is still fresh & shapeable, moisten the pieces and add a little slip to help join them together.
9) Let dry completely then either make a roll over bail for the back ,or drill a hole to attach a jump ring.
10) Smooth any rough areas on the edge (just enough so they're not scratchy). Sponge backed sanding pads are great for this.
11) Fire & finish your pendant as normal (these look great oxidised)
Here's my finished pendant beaded with sterling tribal beads and labradorite 3mm rounds.
Hope you like it!