you might find this easier. You can use it for toe-ups as well.
Try this: start off with the Magic caston, which I and many others find the easiest (on Knitty). Increase until you have enough stitches to go around your foot, stretched a bit. To figure out how many this should be, once you have done an inch or whatever, measure your gauge, you want enough stitches to add up to about 1 inch less around than your foot is. Or just try it on until you have done enough increases for it to feel right. If you are not sure, stop and don't do them, just knit plain, and later if you want them a touch bigger you can increase then. When I'm not sure whether I need to do more increases or not, it's usually enough though.
Your foot will generally be about the same measurement around at the end (before the toes) as it is closer to the ankle, check and see!
The gusset increases are the part where the sock starts to get bigger for the ankle. Or in top-down, after the ankle starts to get smaller for the foot. See this red sock, I have drawn a green V around the gusset increases (this was toe-up). I've drawn it a touch outside the increases, about 1cm or so in you can see the lines of the increases.