I've only been playing competitively for a couple months and since then I've learned the fundamentals( wavedashing, L cancels, SHFFL etc). The one thing I have barely improved on is my dash dance. From what I understand a good dash dance is extremely important, especially for a marth player, but I'm confused on how to go about practicing my dash dancing. What's the general rule of thumb for dash dancing? Is there something unique to Marth when it comes to dash dancing?
It's one of Marth's best tools because of the way the game mechanics play out for him.
Marth has a very long and fast foxtrot (initial dash burst) which allows him to command more space while dash dancing than most other characters. Dash dancing also will create openings for grabs which Marth can convert off of against really any character.
Dash dancing purposefully can be hard to explain but some things you should be doing while dash dancing (with a purpose) are staying at a specific distance (for the most part) in a certain situation, keep your opponent from knowing exactly where you are going, and place your available normals or dash options where you want them.
(I'm sure you noticed but by that I mean when you are in a dash or run you can't do certain things like a smash attack, you have access to your dash attack, jumping, shielding, jump canceled grabs, and grabs. You want to dash dance so you can control space while making your options available to you effective.)
Like if you are DD'ing and say Ganon is approaching you, you want to dash dance at a range where if he tries to attack you you can retreat with a dash away but not dash so far away that you transition from a dash to a run (dash/foxtrot is different from an actual run) and be able to pivot/dash back to where you were to punish him.
That was fairly poorly articulated but it is a start.