How to test out or simulate slashing

I have read a fair bit about slashing but have not actually experienced it. Nor do I want to, at least on a production network!

I have my own application chain on a single machine to play with, but want to actually intentionally create a slashing situation so I can see for myself how it works, in terms of changes to all the different accounts and pools in the system.

How would I do that?

You could set up a validator on the current testnet and have it slashed by either double signing or being offline.

How would I “make” it double sign?