Clip against the hair.
You need to do the outside line (i.e the edge) all in one else you will have a tatty line - it's nigh on impossible to stop a line than continue neatly without leaving a bump.
I basically just take a deep breath and go for it. I usually practise the line I want a couple of times with the clippers off then just go for it, all in one stroke.
When you come to do the other side take a little bit less off if you're not sure - you can't stick bits back on! Again, have a couple of practises to check you've got it even then just go for it.
Remember, people can't see both side at the same time so it doesn't need to be perfectly even. And it usually turns out suprising even anyways.
Always do less than you think - if you have a horse like Pink, they don't grow their coat back until the next spring, so yu're stuck with what you're got.
And DON'T keep taking a little bit off hear, a little bit of there - you end up with a bald horse
Just leave it - once it's grown in a little it'll look just fine.