It is following a picture that looks like this:
Courtesy of Crochetpoet.
Every box in the graph represents one stitch; you choose which colors to do.
Now, typically this sort of thing is done with knitting, but I've heard claims that you can do it with crochet, too.
These claims are lies.
Most crocheters understand that when you do a stitch, the rows are not stacked one on top of the other neatly, like in the chart above. (In knitting, they are.)
So what you end up with is something like this:
Front of sample.
Back of sample.
Mind you, the primary use I have for these charts is to make an afghan, which means that the back of the square can't have wandering ends (so you have to keep the yarn tucked inside with the tapestry stitch).
I did this swatch for my History of Magic homework at the HPKCHC Ravelry group. My exact Ravelry notations:
I loved looking up all of the designs for the Geek-Along afghan challenge. I’d never tried following a graph before…and now I never will again.
I gave up on an original GAL square, instead choosing to find a simpler image through a random Google image search.
Clearly graph designs are meant for knitted squares, not crocheted. The stitches are too off-set and messy for any sort of precise design, so here is where I abandon my attempts to use graphs as a 1x1 representation.
(This is the reason my Minecraft plushies are 3x3 stitches per pixel--they just look too derpy otherwise!)
This brave attempt at following a graph qualifies as my February 2015 History of Magic homework assignment.
And I'm pleased to inform you that over the weekend, I completed everything on my original February task list from Feb. 5th.
(So of course I made an even longer task list that should take me through the end of April.)