Insert hex driver around the jam nut. Be wary of scratching the surrounding output jack, it can be a tight fit.

Give the jam nut a few turns until it starts to tighten.

Be sure to not overtighten it, doing so has the potential to sever the connection wires underneath.

When you've tightened the jam nut, test your guitar by inserting the cable and turning on the amp. If the feedback remains, continue this tutorial for more troubleshooting. If it's fixed, play your favorite tune!