Stop at the end of every row and count your stitches. One of these days when you're much more used to knitting, you'll only do that every once in a while, but for now, check every row. That way, if you're gaining stitches, yoú'll catch them and fix them before you get a long way down the road.
Besides the accidental yarn overs, one really common cause of added stitches is working so tightly that the previous row pulls up and looks like more stitches. Since you're working with big yarn and needles, the problem can be more annoying, but also easier to see and fix.
Another thing that helps new knitters, although it won't matter on this pattern: when you have a project with a right and wrong side, like a sweater, use any kind of marker you need to mark the right side/public side/outside of the work. When you lay your work down, try to stop at the end of a row. If you can't, try to mark the direction you were going. Especially when you're new, it's easy to get turned around. If you have scrap paper and a big safety pin, you can even make notes to yourself--"Row 22 is finished" and pin that to the work so you remember where you left off.