Graph Afghan Patterns
Graph afghans are usually done in afghan stitch, but they can be done in single crochet. You just count each square as a stitch and change colors according to what stitch you are on. To make an afghan out of these patterns, you will need to figure out about how big you want your afghan to be, then figure how many stitches you crochet to an inch using whichever hook you want to use. So if for instance, you crochet 5 stitches to an inch and you want an afghan that is about 48 inches wide, then you figure you need 48 x 5 or 240 stitches across. Then you count how many squares across the chart is and subtract that from the 240, then divide the answer by 2. Then, you will need to add that figure as background to each side of the chart. You will need to do the same thing for length to figure how many rows of background you need to make before beginning the chart. The afghan stitch works well because you never turn your work, you are always working on the right side. With single crochet, you turn every row, so you would need to work across the chart from left to right and then the next row from right to left.
To help better understand how to crochet using graph patterns, click on the following link: http://www.geocities.com/ldiznemom/counts.html
Making a Graph
It really isn't too hard to make a graph for most of the logos. There are a couple of ways of doing it. One, you can get one of the programs that there are for making cross-stitch graphs and import the logo into it. You just need to make it for fewer stitches per inch and fewer colors. There are some pretty good programs you can try out for free if you are interested. Go to http://www.download.com and search for cross-stitch programs. Digi-stitch is a good one.
Another way is to get some clear graph paper (I think you can buy it in some craft stores) and just lay it over the logo and trace it. You would need to make the picture of the logo fairly big to get a good graph for crocheting.