Any backstitching should be done after all the cross stitches in the area are complete. The number of strands to use should be given in the chart instructions. Most often a single strand is used.
A common way to start and end the thread is to run it under four or five of the existing cross stitches on the back if they are right next to where you want to start. You may choose to whip stitch around the second or third stitch as you are running under. This helps to lock the thread in.
Backstitching can be done left-to-right, right-to-left, top-to-bottom, bottom-to-top, or even on a diagonal. It all depends on where the outlines need to be. A backstitch from left-to-right would go like this (up on the odd numbers and down on the even):
To turn a corner without leaving a diagonal on the back side (up on the odd numbers and down on the even):
Some people prefer the double running stitch (also known as a Holbein stitch) to a backstitch. This is especially true if the backstitch will leave them stranded in the middle of nowhere. To do a double running stitch, go forward doing every other stitch (up on the odd numbers and down on the even):
Then come back, filling in the gaps:
To keep the line from looking staggered, be consistent on the return trip. Always come up on one side of the stitch that is already there, and go down on the other side. For example, come up above on stitch 7 and down below on stitch 8.