you will arrive from co flight in terminal c. I suppose you will get out through loading bridge in the satellite so you need to catch a skyshuttle back to main terminal c. once you're out you need to go down 1 floor and follow "transit" direction. Go trough security controls turn left and follow direction c20-33. Catch the skyshuttle back to where you were. 1 floor up in front of gate c20 and dolce e gabbana shop you can find alitalia transit desk where you can check in for your az flight to Tunis.

That's it