Knitted garments often include sections that are mirror images of each other, such as right and left cardigan fronts or right and left mittens. In order to save space, and therefor to reduce printing costs, printed patterns often instruct the knitter to knit the second piece the same as the first, but to reverse the shaping. This is done by looking at what was done there and working the same actions on the opposite side of the work.
Knit your first piece according to the pattern directions. Pay special attention to any instructions that are worked on only one side of the knitted piece.
Cast on or pick up the necessary number of stitches for the second piece. Any decreases that were worked at the beginning of the row on the first piece will now be worked at the end of the row on the second piece at an equal distance to the edge. For example, if the first piece instructs you to knit one, decrease and work to the end of the row, on the second piece you will knit to the last three stitches, decrease, and knit the last stitch.
Work any bind-off stitches only at the beginning of a row. If, on the first piece, you bind off a few stitches on row 15 with the right side of the work facing, you will have to bind off on row 16 with the wrong side of the row facing when working the second piece.
Place markers at an equal distance from the other end of the row, if instructed. A mitten pattern may ask you to knit 10, place a marker and then knit 24 for the first mitten. For the second mitten, reversing the shaping will mean knitting 24 stitches, placing the marker, and knitting 10 stitches.
Make a quick sketch of what you did on the first piece to make it easier to visualize what you need to do to shape the second piece.
Use a different method of decreasing on the second piece in order to have mirror image decreases. Use right-slanting decreases such as a knit-two-together on the left edge of the knitting and left-slanting decreases such as the "slip one, knit one, pass the slipped stitch over" decrease on the right edge of the knitting.
Hold your piece up to yourself if you are unsure about which side of the work the armhole shaping, or neck shaping, should be on.