This is an error I see all the time and it is really very simple to remember the difference.
When a word has been shortened we use an apostrophe to show a letter has been omitted. In the case of ‘it’s’ the original word was ‘it is’. However ‘its’ with no apostrophe shows possession, i.e. that something belongs to someone.
“The dog chewed its bone” (the bone belongs to the dog thus ‘its’ is used to show possession)
“The dog chewed it’s bone” if written out in full would be “The dog chewed it is bone” which makes no sense—thus it is incorrect!
Yours in crisp, error-free writing,