98 Accord Alarm on/off instructions

I have no key fob for my 98Accord. The alarm went off, and, after disconnecting the battery, I found the instructions to turn it off. That’s fine now.

However, I don’t know how I turned it on. The instructions do not explain that.

Also, what triggers the alarm when it is armed?

The car has an aftermarket radio installed, so there’s no valet button.