This is not intended to be the most efficient method of getting them all in one playthrough.
All warhead locations (feel free to leave better directions):

0:00 - 0:42 skip this
0:43 From the Green and Red Buses south of the Cemetery, head due west (past Pets Grocery Food) until you reach the 1:35 green REPAIRS building with the tire on it. 1:48 The warhead is just north of the Repairs building. This also happens to be just south of the downed helicopter, near the Stepping Stones.

1:59 From the back tire of the Crashed Log Truck, cross the street due south past two lamp posts and through the alley. 2:08 The warhead is just west of the alley.

2:17 - 2:34 skip this
2:35 The warhead is north of the downed red bus in the NE corner of the map.

3:05 About 13 seconds west of the bus you will find the Yellow Highway Traffic Information Sign between two roads connected to blocked off tunnels. From the sign, head south for about 45 seconds (on either side) until you see 3:58 the warhead between the two roads.
Alternative directions (not shown in video):
From the SE corner of the basketball court, head due east along the sidewalk and crosswalks, curving around the Hot Pizza building, and continuing east.

4:30 After heading south of the last warhead to the opposite end of the tunnels at the southern wall, head north about 12 seconds and west about 12 seconds to DINER with the hamburger statue/sign/thing. 4:57 from the Hamburger, head due south along the three half-buried pipes. 5:07 The warhead is east of a Quonset hut/barracks building with the torn roof, behind a military truck.
Alternative Directions to DINER (not shown in video): From the southeast corner of the basketball court, cross one (or two) street(s) east, and one street south.

The Basketball Court is, from the Southernmost Westernmost absolute corner of the map, about a ten second walk east and another 15 second walk north.
The Cemetery is, from the Southernmost Easternmost absolute corner of the map, about a 33 second walk north.