LOTS OF BAD ADVICE IN HERE
"Up throw/Down Throw to Up air is one of the most common combos for marth, because of how easy it is to do or learn, so you'll see most marths doing this combo. Basically, if you can do Grab, Up Throw and Up air in a row, you can do it! This does about 13%, but if you do Down Throw instead of Up Throw, it'll result at 14%. To increase the damage you can do your grab attack, but do it quick otherwise they'll break free. A tip when using this, is that you should always go for down throws instead of up throws at higher percents.
To avoid this move, air dodge just as they are about do up air, or if you are a character like link, use your projectiles to space you and your opponent away, as marth has no projectiles.
You can air dodge, to fast fall, to whatever you want if you want to punish them."
Don't ever use Uthrow unless it's for killing or for setting-up Counters on really really bad Bowsers. Uthrow doesn't combo. This is common knowledge. A grab attack is a pummel. You should never go for Dthrows at high percents. No one will ever air dodge. Really? At high percents (before Uthrows kills) you want to throw people OFF-STAGE.
"Up Throw/Down Throw to Back Air.
Not a commonly used combo, because most people Air Dodge or DI out of it, but if they don't you can follow up with a F-Air or F-Tilt Which is around 21% for the whole thing. But small characters will most likely DI out of it.
Avoid by air dodging.
Punish with a up tilt, up air or up smash."
Again please don't use Uthrow. You should never do a Fair after that either. Fair for some stupid reason doesn't hit above Marth.
"And probably the worst one:
Up Throw to N-air. Does around 11%. Kinda useful, but better if you use any of the 2 above. DI out of it, punish with down air, if you're still in the air."
Nair is as thin as paper. NO. And never punish with Dair in that situation. Or almost ANY situation. Don't even Uthrow. OMG.
.......
"So, let's start with my fave.
Dthrow, Bair, Fair, Dthrow, Bair, Fair, Fair, Dair, or what i'm going to call it: Down Throw Ultimate Combo (DTUC).
This will take a while to execute, but with practice, you know it like the back of your hand. You will have to be quick though, so practice in slow motion. (From 0%) You down throw, to back air, to follow up with a forward air, you should do this quick and now be on the ground. Either, you can do a smash, or you can grab and do this: Down throw, back air, forward air, forward air, and if you're off the stage, down air (Just note there it a very small chance of you doing this.)
Please note most small characters like kirby can DI out of this.
If you encounter this combo, just DI, no matter what."
Please stop. Instead of having people memorize this ridiculous string of punishable crap, you are much better off telling people to wing it.
......
"A extended version of the upthrow to upair, is just following up with a back air, and maybe a fair. It's better to use DTUC, but if you can't do it, it might be better.
Note it's easy to DI out or punish this, so learn DTUC."
DON'T LEARN DTUS (s for set-up since it's not a combo). Like OMG. Marth's aerial strings come natural. You should literally wing it.