now number 2, how do you decide which direction for each strike of it you should use?
The 4th hit of DB is generally the most important. You'll almost always choose which direction to use based on who you're fighting though. For many characters, you'll use the Up direction because of the enemy's weak or laggy Dair. ROB is a good example of this. His Dair is laggy and makes him vulnerable to juggling. Most of a good Marth-game against ROB consists of Up-directed dancing blade combos. Snake is also someone you should be juggling.
Keep in mind, however, that the 4th hit up has less range than the 4th hit forward. At high percents, Up sometimes misses due to its short range. A solid hit is always better than a whiffed attack that may leave you vulnerable. You'll know which to use once you see how not-very-far the 4th hit up reaches compared to the 4th hit forward.
Down is generally used based on spacing rather than your opponent's character pick. It's multi-hit, so humans can DI out of it. Using Down when you're too far away or too close to the guy usually ends up with you hurt, assuming the guy DI's in the right direction.
Note: the 3rd hit directed Up sometimes sets up for a tipped 4th hit Up at low percents. Snake and ROB don't enjoy this.