I practiced using both 20XX and Uncle Punch's Training Mod.
20XX Settings:
DI = Random
SDI = Random (Chance = 5)
Always Tech = 25:Forward/25:Backward/75:In Place
Getup Attack = 25%
Shield = 5-25
Neutral Offense = 100
Neutral Defense = 50
SHFFL = Default
OoS = Random or Grab
Reaction = 0/Exact
Tech Chase = Shine:5/Spot Dodge:10/Roll:10
L-Cancel Window = 3-7
Combo Counter set to Timer.
Overlays for IASA, Hitstun, Hitlag, L-Cancel Success and Perfect Wavedash.
I mainly play against Fox/Falco with various characters or practice platform tech on Hyrule/Dream Land/Pokemon Stadium/Battlefield when I'm having a slow day.
On other days I speedrun through Uncle Punch's training options and once I've completed all of them I cycle Ledge Teching, Ledge Dashing and Shield Dropping.
If I feel up to it -- I use Combo Training against various weight classes and tinker around and make up combos that work on Survival/Combo DI +SDI.
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http://melee.guru/characters/marth.html
I also spend some time mastering pretty much everything this website has to offer.