Definitely read the guide, but here is some deeper explanation and more ideas.
The key to big damage with U Sub is setting up anti air clones. For instance, anytime someone jumps at you, do a HP can cancel it to a clone. Even if they connect a hit, it's worth the trade off. Depending on the character and location you can get 35 to 50%+. You can also gain some extra damage in the corner after his autocombo.
Breaking mid 30s with him is tough and you need to know when to do certain combos and when not. Generally what I do is always get them as close to the corner as possible because even if you mess up the combo at the end, or aren't close enough, Sub's corner tactics will almost always ensure him the win. There are some things you need to remember about corner combos extra hits.
Vs anyone in the corner you can get a slide or a ducking LK after Sub's auto combo, but against some characters connecting for instance, a ducking LK sets him up to be free countered. Against Kabal, the recovery of the D+LK at the end of the combo will be extended if Kabal cancels his stun on the last frame to a spin, and then Sub will take at least 56% if the Kabal player is aware of the scenario. This will also happen vs Robot Smoke's teleport uppercut.
Vs Sheeva, Sub can connect a free standing HK after his autocombo, so if you freeze her near the corner, run her in, SUJK, 6 hit combo, standing HK = 42%. Against the Robots and some other characters you can get a standing LK which will be 40%. It's useful to add extra damage at any time with Sub because with him every bit counts, even if it's blockable. This is a reason why you should often leave the last hit of his auto combo out mid screen and choose a mix up free, or blocked hit, it also keeps the opponent closer to you.
Something else to recognize is when your opponent is and isn't on the ground during an anti air scenario. Generally, if your opponent is frozen the instant they touch the clone in air, they are frozen in the air itself. If they sorta land on it and don't freeze immediately, but are still an air attack, they are on the ground. Often times in matches I pick up on this and will do a SUJK with a 6 hit combo and the other player asks how I knew they weren't in the air, and I usually respond with "I just do."
In a scenario vs a male ninja, when you freeze them in air, it's a good idea to try and keep them as high as possible, so say they land on a clone, hit an aaHP, and cancel to freeze as fast as possible so they will be frozen with a more verticle pose than horizontal. This makes aaHPHP JK slide mid screen very easy, for 43%. If they are more horizontal you can opt to do aaHP, run cancel, walk forward, aaHP, JK. There is also always aaHP, glitch cancel to freeze, but we have come to discover that glitch cancels are not always available. It is similar to the random predetermined simultaneous collision priority but doesn't seem to coincide with it at all. It seems like it should though. GCed aaHPs will add 6-7% to any non DPed combo.
Even more specific, is what punches you should use depending how high an opponent is, or who it is. Against a male ninja, aaHPHP might not work, but aaHPLP will, or in another situation vs a very low to ground Jax, aaLP,HP will work.
Another combo to try is freeze in air, aaJK, freeze, which is tough to time, but it can allow an aaHPHP, JK, slide for 52%. Also try and time freezes on grounded JKs because there's about 33% damage available in that.
lol yeah ^^ dude, When I played on live my friends 360's UMK3, I would sometimes get thrown into the clone, however on the DS version it hasn't happened yet I don't know if it's because I play much better on the DS version or I can move faster or the better control or what lol.