As the name denotes, stem stitch is used for lines, outlines and stems of flowers.
First draw the design you want to work on; i.e a line similar like a stem. Now bring the needle up from the back of the fabric on the line you want to stitch.
Make a stitch forward and bring the needle up a little to the back of the first stitch make a stitch forward and bring the needle up, a little to the back of the first stitch. Pull the thread through the fabric, make the second stitch forward, bringing the needle out a little to the back of the second stitch. Be sure to keep the thread below needle. Repeat the process and complete the whole stem.