Make sure the clay pot is clean and smooth. Sand off any rough spots and wipe clean.
If you're going to paint the outside of the pot, the paint will seal it, so you only need to use sealer on the inside. If not, you should use sealer on both the inside and outside of the pot. Brush sealer on first and let it dry.
Paint the outside of the pot with acrylic paint. Even if you don't want to paint the entire outside, you might want to paint just the rim. If you want to achieve a "color wash" look, you can either thin the paint (craft stores carry products for this purpose) or apply the paint and then gently wipe some of it off with a paper towel while it's still wet. Allow the paint to dry.
Cut out flower and foliage pictures from seed packets and plant catalogs. Look for photos with distinctive shapes and lines. Play around with different ways of cutting them out; squares and rectangles can be boring so try angles, circles, or cut around the leaves and flower heads. Even thin strips work.
Begin piecing the pictures together and overlapping them into a kind of "quilt" on the pot, from the bottom up to the base of the rim. Extending the cutouts up and over the rim can be troublesome, so I prefer to leave the rim as is. If your pot doesn't have a rim, go right to the top. Spread paste on the underneath side of a cutout, then set it in place. You don't need to wait until the paste dries to overlap other cutouts over top. When the pot is covered, brush paste or Modge Podge over the entire pieced surface. It will dry clear.
Once the surface is dry, if you want to add a little hint of the base color, you can brush acrylic paint over all or portions of it and wipe immediately with a paper towel. Let dry. Brush a coat of sealer over the outside of the pot, including the bottom. This may not be necessary if you use Modge-Podge, which has sealing capabilities, but it's an added layer of protection, especially if the pot will be placed outside.