By road

From Grenoble

Take the N85 (route Napoleon) to Gap, then take the N94 direction Briançon to Guillestre.
Or
Take the N91 (Lautaret) to Briançon, then the N94 to Guillestre
 
 

From Marseille

On the A51 Motorway to La Saulce, then direction Tallard and Briançon (N94) to Guillestre.