If you decide to use a different yarn for your motif, keep in mind that if you plan on laundering your item, the two yarns should have the same laundering requirements.
Here are some photos, and a video of duplicate stitch. You can chart out your own design on graph paper, or try taking a look through a book such as the Harmony Guides Colorwork Stitches, or Vogue Stitchionary 3.
Basically, duplicate stitch is just tracing or covering up your stocking stitch Vs with another yarn. For your first stitch, you will want to leave a short tail, about an inch, and hold it in place with your finger while you finish the first stitch, just to make sure the yarn doesn't get pulled all the way out.
While doing the heart in my sample, I worked from left to right on the first row, then right to left on the row above, and so on. Try not to jump around here and there too much, in order to keep the back of your work as neat as possible.
Also, don't pull too tightly on your yarn, as this will cause your fabric to bunch up and it won't look pretty at all.
To weave in your ends when you are done, just tuck the tails under a few of the stitches on the back of the work.
Come up at the bottom of your V.
Cover the right leg of your V. Slip your yarn needle under the two legs of the V above the stitch you are currently working on.
Cover the left leg of your V. Push the needle down into the same spot you came up at in Step One.
Repeat this process until you have drawn your motif onto your project. If you make a mistake, it is really easy to fix. Just work the steps backwards to remove the stitch(es) you don't want.
Here is the video. Happy Knitting!
If you like the nails, you can find the Gradient Tutorial HERE