Lets suppose that I would like to reduce $3-SAT$ to problem $A$ in order to show $NP$-completness of $A$.  Then, what should I do ? (I am confused after some thoughts)  Let's get some instance $I$ of problem $3-SAT$ (so $3$-CNF formula) and transform it to $I'$ - instance of problem $A$. Transformation should be done in polynomial time.  $I$ is satisfable iff $I'$ is solvable by turing machine for $A$ ?  $3-SAT$ can be solved iff $A$ can be solved.Should I show 1. or 2. ?
Reduction - what way is correct?
complexity theory;reductions