Don't. they don't work properly and will leave you stranded.
If you must, then get into one and right click on the boat. You get a message "You have the helm, Captain". You get this even if your head slot is empty.
Assuming you were facing the front of the boat (always useful) you may then move it using the arrow keys.
However every time I've tried this they will not cross a zero coordinate. They get to it and then stick and I end up swimming the rest of the way. I haven't checked lately but I'm sure such a minor problem is way down their list of things to fix.
I'll try again and see perhaps. Not much time lost nowadays with gnomes instead of the big boats.
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Wherever I go - there I am.