1. For the arrows, take a look at this image. The little red angle is basically the exact same, just pasted in different places and rotated to fit. The angle itself is the same. It can sometimes be a bit misleading, since everything is getting smaller as we look farther back. By looking at the angle however, you can see that it actually does remain equal, and therefore the gap itself is not getting narrower as we look farther back. If it comes down to thinking of it as though things are getting narrower exponentially, that works - though it's technically not exactly the case.

2. Don't draw them as veins - veins are specific, they branch in a particular way. But if you mean drawing lines coming off the flow line/spine of the leaf, like this, then that's fine.

3. All lines that run along the surface of a form are contour lines - with the organic forms they're drawn in a specific way because that surface is fully rounded. Lines you draw on the surface of your leaves will, of course, follow that surface. Just keep in mind that you're not drawing veins here, you're drawing artificial contour lines.

4. To be quite frank, it's something you'll get used to. What it really requires of you is to study your lines more closely to identify what is what. You will make mistakes - that's normal. But it is something you'll get better at.

I think a few people are getting a little confused by aspects of some of the demonstrations - I'll be making steps to replace and correct them in the near future, but for now, all I can do is correct things in my critiques.