Please check answers to the following related discussion: https://www.researchgate.net/post/Are_there_any_new_ideas_to_get_graphene_from_graphite_by_fast_and_fully_scalable_methods
By Mechanical exfoliation. It is the simplest method and you can get the best properties. Check also this article of Nature magazine: "Large-scale pattern growth of graphene films for stretchable transparent electrodes" (Kim 2009)
If you want to get graphene sheets, you can follow Mauro's suggestion.
If you want to get graphene oxide sheets, you can oxidize graphite flakes first and sonicate the oxidized graphite in water, which will give you graphene oxide suspension.