QUOTE(Jason_piano @ Mar 2 2008, 11:49 AM)

- never double the leading note as it always wants to rise
A bit more about the reasoning behind this one - the leading note wants to resolve to the tonic (in the same part). If you double the leading note, you've got an octave (or a compound octave). As both instances are going to move to the tonic, this means you'll get consecutive octaves.
A few more rules:
Don't double the third in a major chord.
Avoid overlapping and crossing parts.
Don't have gaps of more than an octave between adjacent parts, apart from between T and B.
Don't try to take all these rules in at once. You need a step-by-step intro to harmony, with explanations about each rule and what it means and how it affects what you can do. There's no point knowing that you mustn't have consecutive 5ths unless you know what consecutive 5ths are! (That came up on your other thread yesterday.) So take it one step at a time and make sure that you understand what the rules mean, not just what they say.
Then you need to build a small collection of standard progressions. On the whole, you want to avoid using chords like iii, vii and second inversions outside of stock progressions.
Once you've seen them all in small examples, you can start to think about how to apply them to real situations.
Start with hymnbook harmony and once you're confident about that, get a copy of Riemenschneider.
When you come to doing Bach chorale harmony, the fact that Bach breaks the rules is no defence. What you're expected to do is produce something that follows the rules.
Don't try to do figured bass realisation until you've got the hang of hymnbook harmony.
The figures do tell you exactly which notes you need to use, but you need to be confident about how to arrange them in ways that don't break the rules.
Eventually, you're also going to need to be able to work out which notes are harmony notes and which aren't and how to add melodic decoration for yourself, but you need to get the basics sorted out first.
T.