Read this paper.. its a good start overview of graphite exfoliation

http://wings.buffalo.edu/academic/department/eng/mae/cmrl/Exfoliation%20of%20graphite.pdf

There is a lot of know-how in the preparation of expanded graphite, but you can try it in the following way: dispers the graphite in H2SO4/HNO3=3:1 and shake it for about 30 minutes untill the colour of the graphite particles changes to blue. Then filtrate it, wash it with a small amount of water and put it in the furnance or an oven at 1000°C.

you won´t be able to avoid the production of acidic vapours because there is a lot of acid intercalated in the graphite. By washing with an excess of water, you will continuously destroy the intercalation compound (GIV). but this GIV is absolutely essential for the expansion. therefore, you should do the expansion in a hood or wear an appropriate gas mask.

The degree of expansion of expandable graphite EG can be increased by supplying shock energy. For that you can place small quantity of EG in preheated furnace around 1000 C in normal atmosphere. If you heat the EG slowly, the expansion will reduce. For higher degree of expansion, first heat the furnace to 700 to 1000 C and then place EG inside the furnace.