Is it the same model saddle as your old one? If so, then mearsuring the position on the the rails is an easy way of getting it in the same spot. However, if it's not the same, it's quite possible that the rails will be in a slightly different position to the old one. You're better off to find the centre point of the saddles and measure the distance back from the handlebars.
The degree of difficulty in getting the angle right depends on the seatpost head. Single-bolt types usually have a series of notches on the clamp body to set your angle in; it's just a matter of getting the right one for your preferred angle. Two-bolt "microtilt" posts are a little bit tricky & require a certain amount of guesswork. Tighten the front bolt first so the nose of the saddle is a couple of degrees down, then tighten the rear bolt. If the saddle then tilts back too much once the bolts are tightened properly, loosen the rear & tighten the front a bit more. It's just a bit of trial & error.
Generally speaking your saddle should be pretty well horizontal, so if you can sight it with something level behind it, or place a spirit level along the top to get it right it helps a lot.
Just ride the bloody thing and enjoy yourself!