The syntax is as follow:
>Cooldown Name Time
Name = however you want to name it, keep it short though.
Time = how long the cooldown lasts in seconds
Tip: always add 1 extra second there, it's the time it takes for the gump to load, only work around I found to make CD times match the gump.
Let's see a practical example:
This is my ]roll hotkey. To add a cooldown to it, I inserted a new line saying the syntax above. I named it "Roll" and set it to last for 11s (the actual cooldown is 10, but we need +1s to display right, you'll understand when you set yours up).
The result is this:
That is a bar and it will go down every second. It's very useful to know when an ability is about to come off cooldown, so you can prepare ahead, instead of just waiting for the message.
You can set it up for anything, not just abilities. For instance, I have a cooldown for certain potions and skills. You just have to time it to find out how many seconds to add.
When you have several cooldowns going down at once, it looks like this. Notice how the bars change color to indicate how closer they are to complete.
Also, you can "call" razor macros in the client and add a cooldown in there. Like this:
The "RazorMacro" option lets you call for that macro (make sure the macro name is written as it is in razor) and you add a line for cooldown.
As a last note, the only downside (if we can call it that) is that the cooldown gump resets to the middle of the screen every time you close the client. So you will have to reposition it on your screen every time; a small price to pay for such a good feature to have.