Since the formation of diamond from Graphite is endothermic and the entropy decreases for this process, how is it possible to make diamond?
Diamond is significantly denser than graphite.

atomic densities:


$1.14  \times 10^{23}$ cm$^{-3}$ for graphite
$1.77  \times 10^{23}$ cm$^{-3}$ for diamond


This suggests that higher pressure would favor the formation of diamond.  Indeed, as the following phase diagram shows, diamond is the most stable allotrope of solid carbon at high pressures.  Temperature has little effect, as long as the pressure is high, diamond is favored over graphite at all temperatures up to the point of liquefication.