The trunk likely has insects in it, so the way to kill them is with heat. First, start by rinsing the dirt off the trunk.
Mark out the locations where you want to place the succulents, using the pastel or chalk stick.
If you think you need some depth, then create some depth using a Dremel with a cutting bit and just hollow it out a little more in the areas you have marked for planting the succulents.
Since dry moss is very brittle and difficult to manipulate, making it a little moist will allow you to work with it more easily.