I throw pots on a kickwheel and most are decorated whilst still damp by dipping in slip and then combing with various brush/comb type tools I make with bamboo and stiff bristles. The mark making process both removes the slip in places, and thickens in others. Later when fired, these marks interact with the glaze they are covered by.
I also make pots with slabs of clay, rolled to the right thickness with a Slab Roller. During this process I inlay porcelain and iron rich clays into the slabs. Later when fired, the glazes interact with the inlays; sometimes revealing the exact shape I have chosen, at others they can drift downwards and affect the inlays below.
Both of these ways of making keep evolving from one firing to the next, each informing the next making cycle.