Easy and simple but takes time to grind. Killing zombies etc gives scrap over time, so if you've cleared a large area of zombies, there will be quite a bit of scrap laying around. Some of them light up and others don't. So run around where the dead zombies lay and you will pick up scrap, from what I remember I believe it is 30 scrap per piece you pick up. Then go spend them on unlockables at shelters like map locations of keys, newspapers etc. You can also purchase clothing unlockables etc or purchase vehicles/weapons etc, even food, I believe it was 600 scrap per food item, but I've never personally bought any due to the fact it's so easy to find food around the game world, including free food in the shelters. You can also find scrap in small plastic box containers that usually shine up when you are near it, these are usually at random places but can also be found in cargo trucks when you open the back of them or inside an obscuris container, these are usually marked as an event too. Simply take out the mercenaries and open the container, almost always is a box of scrap with other items there. Some shops like a gas station has an ATM inside, go up and press 'E' at the ATM and you will get a large amount of scrap, usually just above 4k scrap.