Start with a faux pumpkin from the craft store. Tape off the stem if you like the color, then apply chalkboard spray paint in fine layers. You may need to apply 3 or 4 fine layers of paint in order to avoid runs and drips.
Once you've painted your pumpkin, paint on leaves and curly vines with acrylic paint.
Then write "Happy Fall", or "Happy Halloween", or "Welcome", or how about: "Happy Birthday"!