Hello, actually it´s quiet simple. Once the gene (DNA) is transcripted into RNA, the iRNA binds into the RNA (from the gene) and a protein (RISC) recognises the iRNA-RNA and degraded it.

DNA is the guy who has the car….drives it, steers it…does what he wants……
RNA is the guy who built the car and DNA added accesorities..
RNA is a happy chassis.
DNA can’t help changing and evolving to a future goal, but he gotta drag our friend RNA with him….Because RNA ain’t talking. Just giving hints…..and chemical signals.
RNA is the boss, doesn’t have to speak…and DNA knows it.
DNA and RNA agree!!! They are the basis of life…..anywhere -[— anywhere

It starts with a long piece of double stranded RNA (dsRNA), produced either by the cell itself, or injected into the cell by a virus. The dsRNA is chopped into short fragments (about 19-22 base pairs long) by an enzyme called dicer. These short fragments are called short interfering RNAs (siRNA).

The siRNAs separate into single strands and combine with an endonuclease enzyme complex called RISC (RNA-induced silencing complex). One the single strand siRNA is bound to RISC, it base pairs with the target messenger RNA (mRNA) and cleaves it. This prevents the target mRNA from being translated so the encoded protein is not produced.

This is as simple as explanation of RNAi as you can get.