Nowadays, CRISPR-Cas is one of the easiest tools to use to knock out a gene. You deliver Cas9 (or Cas12a etc) plus sgRNA to cells, which causes a break to be made in a very specific spot. Then the cell tries to repair the break, usually via a messy pathway called NHEJ which often leaves the gene functionally inactive.

If you can live with just partially shutting off the gene, RNA interference is also very popular and can be very quick and easy to do.