The paint shader is accessible from above the 3D preview, the paint brush. It's best to let the preview render till it says finished and than click Pause at the top before you start Paint shader work. Go to Start painting shader and click create new paint shader, then name it, inspect the setting and I always set the brush flow to 1. Use view relative brush. select the area you want to place whatever, texture, displacement etc. and paint awy.
Close paint shader from the same button and go to Shaders Tab. Create a surface layer and at the bottom in the Blend Shader
Assign your Paint shader to that Surface layer with the plus sign beside the dialogue.
Then in the Surface Layer go to the Displacement Tab and with the plus sign again Create new shader>Displacement Shader>Power Fractal Shader and you're set to
tweak.You control the Displacement from the slider in the Displacement Tab which only slides to 10 but you can type in any higher value...100 to start for what you're after.
When you've got what you want you can texture it from the colour tab of the same Surface Shader
The screen grabs are named but not necessarily in order
