You slash just before baking. It gives the bread room for that final *poof* it does in the oven (oven spring). Use a really sharp knife or a razor. Wet it and slash down the bread with the blade at an angle. I do two slashes on mine, about a third in from the sides, holding the blade at about 45 degrees and going 1/4ish inch deep.
I also mist the top (right after the slash) with some water to keep it moist while the oven is working to dry it out so it can expand.
It is a good looking loaf for a first attempt (a really good looking loaf). In the picture there is an imperfection on the top, this might just be an air bubble but could also be a bit of a tear, which happens when you don't slash and give it the room to expand.
Your first time slashing will be ugly and frustrating.
Keep at it and you will get the feel. Do NOT be timid with it. Slash with confidence, if you try to baby it the dough will stick and you will get ridges in your slash.
The bread tastes just as good though, so keep making it and you will get it down.