Ok, as you know cork is a great rock building material. From the humble wine stopper to flat sheets for noticeboards it has stacks of uses. What cork is it you are using? That will determine how you use it.
Wine Cork - Natural
This can be used a number of ways. Mainly people will gouge bits off of it, but you can also cut it into strips and layer them up to build bigger formations.
Wine Cork - Reconstituted
This is made from little bits of cork glued together much like MDF or Chipboard. This are great for creating smooth sandstone/granite 'crowns' of boulders that just break the soil. It can also be used for earth shaping and creating pool bottoms and stream beds. It's a bit of a faff though, as it's glued, so there are easier things to use.
Cork Sheet
This works best being layered up to create bigger rock formations. Make sure that the edge you want to be the 'outside', or visible edge, you rip rather than cut, just to give it a rougher texture.
Cork Bark
This stuff is great. It's the stuff they use to create little hidey holes for reptiles and stuff. You can get it from a pet store for reasonable prices. You can use it straight as it is for rock/cliff faces, or break it up to create lager boulders for bigger bases and display boards.
The best piece of advice I can give though is experiment. Not only with the formations you can make, but also how you stick it. It can be very absorbant in some forms and is better suited to thicker glues and in others you may find that superglue will solve it. Have a play and see what works for you.
Hope this helps,
u_m