Knit two rectangles (with rounded edges if you want to - increase stitches at the beginning of the rectangle, decrease at the end.), one in one colour of wool/yarn and the other in the other colour.
The first rectangle for the body needs to be 1 inch wide by 2 or 3 inches long.
The second rectangle for the shell needs to be 1 inch wide by at least double the size of the body in length.
Of course you can choose to make these sizes bigger or smaller depending on the size of snail you want.
Sew on beads for eyes and antenas in a suitable place on the first rectangle.
For the antenas, take a peice of wool/yarn and tie a knot at the end. Thread onto the needle and go up through the rectangle. Cut the wool/yarn at an appropriate legnth. Tie a knot at the bottom of the wool/yarn.
Sew the rectangle together into a sausage shape.
Sew the second rectangle into a sausage shape.
Sew the second sausage up into a danish pastry shape for the shell, using pins to keep it in place.
Sew this on to the body and you have a snail!