How does printing money cause inflation?
WHY and HOW monetary issue causes inflation.

When a central bank emits money (or decrease key interest rates), commercial banks will be able to refinance themselves at a lower cost. Which means that at the end, these commercial banks will also be more, say, flexible, and thus will stimulate demands (here and there) in the national economy by lending money (here and there). If supplies stay constant, markets will reach a new equilibrium via prices rising due to these stimulated demands $\implies$ inflation.