Assuming your girl is on medications, I would do this during then so she are more aware of what is happening as you would not want to catch her off guard, potentially causing aspiration.
You will want to administer a very small amount at a time as it may likely be thinner than her other medications so might catch her/you off guard.
As for how much I am not really sure, if she got medication 3x a day I would personally administer 1ml of water per time.
You can also make her critical feed more watery, as said above, again being careful to administer it slower.
One thing I also find is that if they are on my lap and I hold their water bottle up to them they will have a drink, so you could offer this during cuddles. I also give it a little squeeze, not too much but just a little extra.