Skim is one of those verbs that doesn't do what others usually do, although in a way if you think of what 'to skim' means, it does make sense, because it means 'go over quickly' or 'take the top off of' and not any direct action, like eat, or read, both of which imply more depth. Skim requires another word to 'work' propery ... therefore to make it 'work' means knowing exactly how you are 'skimming' and what ... so if you are talking about working with a liquid, you would say not 'I skim' but 'I skim off' ... if you are talking about a book, then you say not "I skim' but 'I skim through' ... and you'll soon see that while you CAN occasionally use the word 'skim' by itself, it tends to work much better with another word ... so ...
I skimmed over the ocean in my dreams.
I skimmed down the list of questions, here.
I am skimming my words as I write.
I have skimmed like this daily.
So, remember ... skim, skimmed, skimming and you should be okay, but ALWAYS read that sentence over to BE SURE and look for those 'other words' to provide the modifying objective you both want and usually need to have this word make true sense in a sentence.