1) Make a trigger:

on *:SOMETHING:thetrigger:#channel: {

^ The "SOMETHING" can be like TEXT, ACTION (/me's), NOTICE, INVITE, PART, JOIN etc. Replace #channel with the channel you want it triggered in, or leave it as # for all channels.

2) Make a command for the trigger to evaluate:

on *:SOMETHING:thetrigger:#channel: {
  something here

^ The "something here" can be (for example) msg $nick something, notice $nick something, msg $chan something.

3) Make a closing bracket:

on *:SOMETHING:thetrigger:#channel: {
  something here
}

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A finished Example:

On *:TEXT:!pie:#: {
  describe $chan gives $nick a pie
}

^ describe $chan = /me, $chan = channel its typed in, $nick = nick who triggered the command.