Here's one of the standard reference guides to style:
http://www.bartleby.com/141/
It's a very big subject, but here are a few pointers:
Formal language doesn't use contractions.
formal language uses fewer phrasal verbs.
It uses more vocabulary derived from French and Latin (bigger words, basicall)
It uses more past tenses (remote forms) of modal auxialiries. A formal instruction may well use 'should' instead of 'must.
It uses more indirect language- the passive is more common, making it less personal.
It uses more synonyms and doesn't repeat the same vocab as much.
It is better organised and thought out.
Here's one of my favourite articles about the language by one of its great writers:
http://cscs.umich.edu/~crshalizi/Orwell/politics_and_english.html
Hope that helps ;-)