It is always healthier to use malt vinegar which is usually dark, but you can use either. The white vinegar looks nicer as it does not darken things. I usually use white sythetic vinegar.
It is very important to do it fast while it is quite hot. Once it starts setting the lumps start forming. Use a rubber spatula. Do not worry, do it once or twice and you will get a hang of it.Don't feel discouraged.