Basics: Get shovel (2 in start room), dig the mounds that appear in game until you get the Golden Shovel, the GS icon will replace the regular shovel icon by your name and make a loud 'ding' sound.
After that, while using the Zombie Blood drop, you may see orange mounds to dig. If you get to it and dig it, there will be a perk bottle, these give an extra perk slot that you can then fill by purchasing a perk. Originally you max at 4 buyable perks, each bottle raises that number by 1.
Okay that's the basics, here's the special bit: once you have the golden shovel, but without needing Zombie Blood, you can take Semtex (buyable from wall icon) and throw it at 1 of the 16 locations in the game where the orange mounds may load.
When you get the 1 that will be the 1 that you could see in ZB, it will glow orange when the Semtex lands near it and stays that way until you use ZB.
You can still only dig it in Zombie Blood mode, but now you know where to run to when you get ZB.
Free Zombie Blood drop; Have the Ice Staff (upgrade not necessary).
This next part must be done fairly quickly, the wagons will extinguish, but don't watch for that to happen, just keep moving;
Face Generator 5 from the tank path, on your left is the first burning wagon by the path to G5, shoot it squarely in the side, quickly head right towards the path from the workshop, right there at the top of that path is the next burning wagon, shoot it squarely in the side of the wagon, move on clockwise around the dig site, as you near the path on the Church side of the dig site, you should see the last burning wagon up on your left, shoot it, continue clockwise, head up into the dig site and you should see Zombie Blood on your right when you get up top.
[more cheats for Call of Duty: Black Ops 2 here]