Here's an example of a late apex
http://www.youtube.com/watch?v=vVy_w7Vjikc
Early apex would just be going the opposite direction.
You can see that the turn gets tighter and tighter with the late apex.
Mid apex would have the tightest part of the turn in the middle.
I see that Spa Francochamps is in GRID.. that's my favorite track.
So the first turn is a chicane.. you mostly just try to go straight there. I just take my foot off the accelerator for a split second right before the turn, and immediately floor it again.
After a long straight, you'll run into an early apex. So after you're done braking, this is the slowest point of the turn. Then you'll want to make a right and try to clip off the apex a bit. Then you'll slowly and gradually try to speed up again. You'll first make a right, then you'll quickly make a left again. Just try to maintain your speed and try to build it up as much as you can while retaining control.
Amost immediately, you'll have another early apex right turn come up. Try to go as wide as possible. I try to think of cutting the apex(very important for early apexes), and I often cut it a bit too much even. But this is better than not cutting it enough. You're already going at a steady speed at this point to not need to brake, so just ease off the throttle, and make your turn, then right after you clip the apex, start accelerating as much as you can without losing control.
You'll get to speed up a bit here on this short straight, until you come up to a late apex right turn. Brake down about two gears here. You want to brake just enough so that as you roll through the turn, you'll lose another 10mph or so to make your turn speed just right for the apex. Then after hitting the apex, steadily accelerate out.
If you brake too much on this turn, you'll find yourself needing to build up some speed for the curve, and suddenly needing to slow down again for the apex.
I think I covered early and late apex thoroughly here. For mid apex's, treat it intially as a late apex, letting the roll through the turn slow you down a bit before you hit the apex, and post-apex, it'll be like an early apex turn, where you'll have to gradually speed up as much as you could while retaining stability.
Here's a vid of the track I'm talking about. I just described from 0:20 to 1:08
http://www.youtube.com/watch?v=zSRWCoNvM-k
Here's another vid.. I think this one shows the late apex more clearly
http://www.youtube.com/watch?v=PPTAv9t36IE&feature=related
Uphill and downhill will just affect how early you need to brake and how much you need to brake. Uphill you'll have more traction and you can brake later. So you can not only make the turn faster, but you can brake more quickly too. Basically when it's uphill, you can drive really aggressively, while downhill you have to be extra careful and conservative.