Working with uneven tesserae like millefiori, broken china and ceramic tiles can be tricky. Below we have outlined some methods for achieving a flat or relatively smooth mosaic surface.
Smear thinset bonding mortar onto the surface you are mosaicing with a small trowel or putty knife, to a thickness between 0.3 m.m's to 0.6 m.m's. Press your tesserae directly into the thinset, with the thin pieces having more concrete underneath them. Ensure that you have a damp rag, a container of water and dry rag handy to clean up any excess adhesive and keep the face of the tesserae adhesive free. Note, this method will not result in a completely flat surface.
Lay your tiles directly onto a bed of thinset and use guides (e.g. wooden blocks cut to the level of the mosaic) and a flat surface (e.g. a square piece of ply or large ceramic tile) to level the tiles. Working in small sections level the tesserae with the flat surface to the level of the guide.
The indirect method of mosaics is the most effective method for achieving a flat surface with uneven tesserae. This is because the mosaic is made face down onto an adhesive surface. For more information on the indirect method see our 'How To - Methods of Mosaics - The Direct and Indirect Methods.