How exactly..................

How exactly do you play a gambit if it is declined? Is it bad if your opponent declines it ? Also can you tell me some lines if a gambit is declined?

Which gambit? The questions you ask totally depend on what opening you are talking about. The King's gambit is almost the complete opposite of the Queen's gambit.

ummm a simple king's gambit  what would you play if the pawn is not taken

Here's a typical KGD

Ah looks kinda like an italian game but basically you just develop after the gambit is declined like any other game. but after b c5 can white take the black pawn?

also can black play d6 to defend his pawn ?

White can't take the pawn before the knight moves to f3 because black would reply with Qh4+ and then you lose a rook.