Duplicate stitch is a nice way to embellish or beef up any knitting project....AND it's super easy.
Plan out your design using graph or grid paper. Thread a tapestry needle with your yarn but do not tie a knot. Insert the needle from the work side into the bottom of a knitted "V." Leaving a few inches of yarn (to be sewn in later) on the work side, bring the needle up and over the left arm of the V. Going under the row (leaving the gap in the top of the V), carry the yarn to the other side of the V. From the top right of the V, follow the stitch down and insert it into the bottom of the V.
From here, you can either continue over to the next row, starting in the bottom of the next V, or up, depending on your design. Try to be consistent with the way you move through your pattern - either stitch side to side or up and down. Continue until your pattern is finished. Weave in ends.