The next step to create a mishima design is to inlay slip into the incised design. Generously apply the contrasting slip or underglaze to the pot. Remember that as the slip dries, it looses a lot of volume. This means that if the slip is flush with the pot's surface when applied, it will become a sunken design as water evaporated out of it.
If you are using a single inlay color, you can simply spread a thick layer of slip over the entire surface. Broad brushes or sponge brushes are good for large areas, and smaller, soft brushes work well for small areas.
If you are using multiple colors in your mishima design, carefully apply the slip to the appropriate areas. Use smaller brushes and be careful not to allow the slips to inadvertently smear into each other.
As you apply the slip, be certain to push the slip down into all incised areas and lines. Air can easily get trapped below the slip if it is not forced out as the slip is applied, ruining the mishima design.