This is the primary stitch used in needle lace, but in Brazilian embroidery we use it a bit differently.  Here rows of detached buttonhole stitch are worked over a single straight stitch, or added to the outer edge of other stitches.  The final petal or design element is worked generally stays loose from the fabric except at the beginning join point.  

Begin with a straight stitch as a foundation.  A tapestry needle which has a blunt point will be easier to use for this stitch.  Come up at one end of the straight stitch.  If you are only working one row, work clockwise.  If making more than one row, some have found it best if the final row is worked clockwise, but alternating rows in between will be worked counter-clockwise.  On the example, the first row is being worked counter-clockwise and the second row clockwise. 

Each stitch is worked by forming a loop in the direction of the work and taking the needle behind the foundation or stitch in the previous row and over the loop of thread.  When you reach the end of the row, making the desired number of stitches, reverse the direction of the thread loop and work back in the loops between stitches.  Keep the stitches somewhat loose but not sloppy. 

When extra stitches are required in a row, you may increase by making two stitches into one loop, or by making stitches both into the loops between stitches and the little e that is formed by the stitch.  Some suggest that making stitches in the e's creates a sturdier petal. 

Finally, after all rows have been made, overcast back down the final side to the fabric and take the needle down to the back.  The petal may also be finished by taking the needle down at the tip of the petal, thereby holding it in place, rather than leave the petal free of the fabric.