First you need the two cardigans of two different colors and then you just cut them down the middle. You can measure it or follow the lines of the knit.

Next you can take some double fold bias tape and open it and then pin it to the cardigans so that they are together. (Make sure you have one half of a cardigan with buttons and one half with button holes). Stitch along the bias tape to secure the two cardigan halves together.

Next, you just need to fold the bias tape over so that it makes a nice seam on the two cardigans. You do not necessarily need bias tape, but it makes the seam cleaner and it does not stretch as much. After you are done sewing, just iron the bias tape to one side and you are done!