this is a really broad question, but how do you score in DDC? It's my favorite game in the series, but I'm not sure how to get good scores.
DDC scoring is primarily a combination of canceling bullets for PIV, item collection bonuses, and spell bonuses.
Point item value is increased almost entirely by canceling bullets-- graze is a minor factor, but not a high priority. (SakuyaB also gains extra PIV by shooting focused.) There are two ways to cancel bullets: finishing off patterns, or bombing stage portion/nonspell bullets. SakuyaB can also bomb-cancel spells for PIV, but other characters can't. Try not to finish off patterns with a bomb, since then you lose the manual cancel at the end.
Items are worth an additional bonus (0.5 at 20+ items to 2.0 at 60+ items) and a bomb part if you auto-collect at least 20 items at once. The best possible bonus is 2.0, from auto-collecting at least 60 items, which also gives a guaranteed life part (you also get a life part every 5th bomb part). So plan out the stages to collect as many 2.0s as you can. Since auto-collecting with a bomb still gives the bonus, it's worth bombing for 2.0s since it gives more score and a life part back.
Spell bonuses are rather high in DDC, so stay under the boss during spells as much as possible. ReimuA and SakuyaA also have advanced ways to improve damage (ReimuA: centering the gohei on the boss is more damaging, and shift-tapping adds slightly more damage on top of that; SakuyaA: if you can safely position yourself directly under the boss and shift-tap, the knife explosions overlap for ridiculously high damage). It's often worth losing some spell bonus to wait for a better PIV cancel, though, especially in the earlier parts of the game.
Also keep in mind that additional enemies spawn if you defeat a midboss (except in stage 6) especially quickly, so sometimes there's a trade-off between PIV farming and additional items (for score/extra resources).
As for the clear bonus, it's mostly relevant in Easy and Extra. In Extra you get 40 million per life and 4m per bomb; in the main game you only get 10m per life and 3m per bomb. On Easy it's worth finishing with 8 lives and as many bombs as possible for the 90m+ bonus, but on higher difficulties it's often more profitable to spend your lives bombing for PIV and 2.0s earlier in the game. (Don't bomb for PIV in stage 6, though, since there aren't enough remaining point items.)
Anyway, I recommend watching some scoring replays to see it in action-- try downloading some from
Royalflare/th14 for your preferred difficulty/shot type. I also have videos of
the overall Lunatic SakuyaB world record and
my old Easy ReimuA 873m, if you'd prefer watching something you can rewind.
If this entire post seems like too much to keep track of, focus on the three goals I listed at the top first. Feel free to ask if you have any additional questions.