You can make this stitch by imagining two lines, one on either side of your seam, equal distances from the seam and working between them. Start as you would to make a chain stitch by bringing up your needle, going down again very close to where you came up and taking a bite of fabric. Before you pull the needle through take the thread around the needle so that it will catch it and form a loop or what would usually be the first link in your chain.
Normally you would take only a small stitch to hold the loop open if you were making a detached chain stitch. With this stitch you want to take a larger stitch, approximately the same size as your chain stitch, as seen above. When you go down in that stitch you will want to come up again on the imaginary line on the other side of the seam, even with the bottom of your last chain.
You now make another detached chain stitch. Please note that before you form your loop you will take the needle under the last straight stitch. Carry on in this manner - your straight stitches will form a zigzag pattern.
In case this isn't as clear as it could be, there is a tutorial for this stitch in Sharon B's Dictionary of Stitches. Have fun!