Thin MDF board is undercoated before a paste of Poly filler is spread evenly on the board.
Natural and man made materials are embedded into the paste and just before near set and dry, remove the materials from the paste surface. When fully dry, sand with fine sand paper and seal with a varnish or undercoat.
Use a toothbrush to ink up the first colour into all the recesses.
* Brenda Hartill so generously has produced a reference book and video on this Collagraph Technique.
I was very fortunate to attend a talk by Brenda Hartill at the Bendigo Pottery in 2013 and I saw first hand some of her beautiful prints and her wonderful Collagraph blocks.