Before cutting and gluing the scrapes, let's prepare the background. We have two possibilities:
We can choose to leave the original white background of the piece of pottery,
or paint a different colour for the background.
In the first case, before gluing the scraps, I suggest to wash carefully the object using a washing-up liquid, in order to skim the surface, then dry it with a cloth which doesn't leave hair on the surface.
In the second case, I suggest to use specific colours for pottery, there exist some which can be dried in a normal kitchen oven.
The decoupage technique on a piece of pottery might be difficult for different reasons:
Objects made of pottery are often concave, and they have camber on many sections, for example a soup plate has both horizontal and vertical cambers whereas a cylindrical cup has just a horizontal camber.
That is the problem: the scrap of paper has flat a surface, while the object to be decorated with the scrap has cambers in more than one directions.
We all know that if we want to cover a 3-D object using a sheet of paper which is 2-D we somehow must fold the paper.
So if we glue the scrap on a concave surface small creases will develop.
In order to bypass this problem, it's a god idea to make small perpendicular cuts on the side of the scrap. Or better, let's make this cuts along the lines of the drawing, for example, if we are using a flower figure, let's cut along the petals line, in order to hide this cuts.
In the detail figure, the white signs are the lines surrounding the petals, along which I've made the cutting. This way the edge of the cut will overlap slightly so no creases will develop.
Another problem which might happen when gluing on pottery, is that the surface is perfectly smooth, so it's rather difficult for the scrap of paper to cling onto it, the scraps sort of skate away on the glue coat. Keep the glue less watered down.
If you have choosen to leave the white background, then after gluing you'd better remove the glue in excess on the border of the scraps. I suggest you use a wet paper towel, well squeezed, and tap on the pasted scrap without rubbing it.
Once the surface has perfectly dried, then you can give the varnish coat. In order to keep the shiny look of the pottery, it's better use water varnish without wax, water varnish for parquet might suit, but there are special varnishes which simulate the shiny look of pottery. As in ordinary decoupage, you'd better give several coats of varnish. Don't use steel wool to smooth the surface, it's better abrasive paper, because steel wool soils the colours with a shade of grey.