Your Neutral Toolkit
Combos
Millia's combos rely heavily on launching opponents into the air. Her most common way to do this off ground-to-ground hits is 2H, a jump-cancellable launcher, so combo into that if you're close enough. Millia can also convert into knockdown combos from most air-to-airs and her throw.
Starter > c.S(1) or f.S > 5H > 2D > Forward Roll > H Tandem Top
is a good ground gatling to fall back on if you aren't close enough to land 2H.
If you've got them in the air, you want to find a way to route into Stuff > j.H > airdash > j.2H > H Tandem Top
in order to pull them back to the ground, secure a combo, and begin your oki.
Basic Combo
Basic Combo From Air-to-Air
Basic Combo From Throw
These combos work because j.H is airdash cancellable, allowing Millia to carry opponents to the corner and string air normals together in unique ways. In addition, Millia's combos are height-dependent; j.2H will knock down if the opponent is close enough to the ground, but if you're too high up, the opponent can tech. If you think your opponent might be too high up to be knocked down, cancel into Bad Moon. It leads to worse oki, but it's consistent.
You can route into 2H off any ground normal aside from 5H, 6H, or D normals. In addition, most air normals can gatling into a combo to j.H somehow, so experiment with what kinds of combos you can get off certain hits.What to do after a knockdown
In general, oki after using H Tandem Top (henceforth "disc") will look like this:
The first layer of oki after disc is to either run a mixup, bait defensive options, or some combination of the two. For example, a safe option to check reversals from your opponent, as well as a low-risk low mixup option, is meatying with 2K. You can use 2K once, then block and wait for disc to hit the opponent. If they used a reversal on wakeup, you can block in time, wait for disc to hit them, and get a confirm. If they blocked high in anticipation of an overhead, 2K > disc will combo, and you can once again get a confirm. If they blocked low, disc will give you a lot of plus frames, allowing you to re-establish pressure and run a normal blockstring.
Millia has a variety of mixup options on okizeme, all of which carry varying amounts of risk, beat different defensive options, and have different reward. While a safe, low-risk option like 2K may lead to a less damaging combo, a riskier low mixup that cannot beat as many defensive options, such as 2D, could boast greater reward on hit in the same situation. Mix up your options based on what you think your opponent expects, how much of a risk you're willing to take, and the opponent's defensive habits.