To work duplicate stitch, you need a finished piece in stockinette stitch, a good yarn needle, and some contrasting yarn of the same weight as the knitted piece:
Decide what you want to stitch, where you want to stitch it, and how big it should be.
To figure out how many stitches you have to work with vertically, multiply your row gauge by 2. Thread your needle with the contrasting yarn.
Insert the needle from the wrong-side to the right-side at the base of the first stitch.
You see the yarn come out the front of the piece.
Follow the line of the stitch and put the needle back in at the top of the stitch.
That is, in the row above the stitch that you’re duplicating.
Go under the full stitch (two legs) in the row above.
Bring the needle back out to the right side.
Put the needle back in where you came out.
This location is at the bottom of the stitch.
Go under one full stitch (two legs) in the row below the stitch you’re duplicating, and then bring the needle back out to the front.
Follow Steps 3-6 for each stitch that you want to duplicate.