What you're talking about here is intarsia, assuming that you want to make it in color.
If you don't want to use color, you can purl on a knit background for a textured motif.
There is a video on intarsia, but I would also recommend getting a book on it to refer to.
Basically, you would knit along in your background color until you come to your motif. Then without cutting the main color (mc) you start knitting in the contrast color (cc). Each cc can be held on a bobbin or left hanging in the back. If your colors are close together you can strand them across. If they are more than a few stitches apart you would start a new strand for the next section.
If you want to graph your own, use knitting graph paper. It's proportioned for knitted stitches better than regular graph paper. Just do a Google for it and you can find it printable. I don't remember the site off-hand.
This is a start. There are ways you should carry the yarn and switch colors, but one step at a time.
One more thing. If you do this on a blanket, the back will be in reverse and not look as nice as the front. Intarsia is definitely a one-sided picture.