Using highlighters, I use yellow to mark the symbols of the color I am going to work with next. I have the symbol count in my pattern info, so I count as I go to make sure I get them all. This allows me to plot the most efficient course of stitches that I can through the chart. As I complete these stitches, I go over the yellow with a pink highlighter. Now when I look at the chart, anything that is orange is done.
Baste a grid onto the fabric. Some people like a 10x10 stitch grid. Others just use one horizontal and one vertical line through the center. A variation is to baste a small "ruler" near the edge of the fabric, outside the area of the picture. Be careful to use a thread color that doesn't show too strongly against the fabric, just in case the basting thread leaves little fuzzy remnants behind. Use a thread that can't be pierced with your needle so that the thread can be pulled out easily when you are finished. You can pull the thread out as you reach it, or stitch over it and pull it out later. Some people find that monofilament and quilting threads work well. Experiment to find out which method works best for you.
A word of caution--if you use a grid, do not completely fill in each square one-at-a-time. The squares may show up in your finished stitching because of the change in tension at the end of every row in every square.