wind up your gain (and a newish set of strings helps).

Grab your pick near the tip so that you can touch the string with the fleshy part of your finger or thumb.

fret a note and then pluck the same string whilst lightly touching the string with your finger or thumb.

Try this in a couple of different places (like touching the sting over the top of the bridge pickup or over the top of the bottom of the neck).

They take a bit of practice and you have to be on the ball to remember where about you want to touch the string to get the squeal you want, but that is how it is done !