How do you use touhou skill card?

Answer 1

(Someone please fill in for IaMP for me)

In SWR and Soku, when you customize your deck you have "skill cards".
Therefore, when you add skill cards to the deck it either adds new special attacks, (and replaces the former; for example Cirno's default 623B/C is "Freeze Touch Me". Should you activate a skillcard with 623b/c as its configuration Cirno will perform that attack rather than the default. Then again, most of her 623 attacks are sorta awkward for comboing)
Or it will make stronger/change slightly the attack you already have if it is the same attack. 4 levels max.
To "use" the skillcard you switch to the card in your deck during battle (by default for 1P it is the "S" key, or Z+X) and then cast it by pressing D or X+C.
Sometimes the attack will be performed as you cast it if there is a change. (Only in Soku.)
Notes:
Suwako is a very, very strange exception to the casting of skill cards.
You must know whether they can be activated in air, on the ground, underground, or in crouching state.
Take the time to learn it.