Simply load up the 3rd level of the campaign 'Cliffhanger'. After killing the first two groups of enemies with Captain MacTavish, you both go your seperate ways and you put a ski mask on (from here on you cannot alert, or kill any enemies). In my video I traverse the level as follows (note that I did all of this crouched and walking to minimise chance of alerting enemies):
1) Turn left around the gate, waiting for the two enemies to pass by and be shot by your teammate
2) Climb up to the left and hide next to the boxes that are next to the road (staying back from the road), you must wait for the truck to go past you
3) Cross the road and hug the right side of the wall, taking your next right turn
4) As you turn, have the enemy that is on your right, in your sights so that your teammate can elimate him
5) Turn towards and face the objective marker, head straight there (ignoring your teammate along the way)
6) Plant the C4!
I created this video to help:
Hope this helps!