Back stitch is a simple straight stitch that is great for outlining a picture, or using anywhere that you want to show a simple line.  You can curve the line or keep it straight, depending on the pattern that you wish to create.

Start by pushing the needle up through the work from the back of the fabric, just after the point where you want your line to start (shown in picture)

Draw the thread up and through, leaving a small tail at the back (see 'getting started' section for how to secure the tail at the back).

Push the needle back down through the fabric at the point where you want the line to start from, as shown in the picture above. The needle is then brought back up through the fabric just after the point the the thread comes up from the back, following the line of your pattern. Pull your thread through, making sure that you catch the tail of thread at the back to weave it in securely.  Be careful not to pull the thread too hard and pull your tail through to the front! Your work should now look like the picture shown below. 

Insert your needle back down through the fabric at the point where the first stitch ends and bring it back up a little further along your pattern line, as shown below.

Pull the thread through again, as described before, taking care to weave in the tail at the back to keep your work neat and secure.  Continue your line by repeating these steps until you reach the end of where you want the line to go.  Weave in a short length of thread on the back of your work to secure the end and then cut away any excess thread.