Just swapped mine out a month ago. The trick I found to be the most helpful was to use a scissor jack to hold/lift the unit in place. Being on your back under the sink makes it hard to lift the unit up at the same time you are trying to tighten the collar. If you are removing the sink to reseal, and clean up the counter, (good time to do this) then install the unit before you put the sink back in (seems obvious enough, but don't ask me why I am reminding you to do it)

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