To prepare rocks/stones - wash them with mild soapy water to remove dirt, rinse well, and allow to dry.
For delicate leaves and flowers - if they are fresh, press them between absorbent pieces of paper and heavy books until they are dry, or, iron them gently with a DRY iron on low heat between two pieces of paper.
To prepare sticks or similar items, brush the dirt off with a stiff brush, spread on a cookie sheet and bake in an oven on low heat to kill any hidden bugs, fungus, etc. (I usually use the lowest setting and bake for a half an hour to an hour for sticks 1/4" to 1/2" in diameter)
If you want to use purchased material such as dried flowers, check for a waxy or oily feel and wash them if necessary - these materials are often preserved with glycerin to keep them supple, but excess glycerin or waxy coating can prevent clay from sticking well.
For materials that have sap or resin, such as the pine cone "scales" in the project below, clip the scales/seed wings from the cone with clippers or tin snips, wash the clippings then spread them out onto parchment paper and bake as above. Repeat if necessary to remove the majority of sap.
I'd love for you to join me for more projects & art adventures on my personal blog