The first stage of Blood Harvest takes place in a wooded area called the Woods, surprisingly enough. Like all of the first stages, this one is relatively simple. There is a clear path to follow and it is mercifully short. If you move quickly and efficiently, this should give you no problem. Through the entire level though, you need to keep your eyes and ears open and watch the woods to either side of the path. Special infected can hang out here relatively undetected, and while there aren't any rooftops to leap on you from or pull you off of, it is still possible for a crafty smoker or hunter to trap you in the woods so that they can gnaw on your brains at their leisure.
As soon as you grab your weapons and health kits, leave the campsite and head down the path. Stick close together and there should be no problem. If you get swarmed, kneel down back to back – don't go into the woods looking for a safe corner. Stop briefly at the picnic table and look for pills or explosives, but don't wait too long. It's a great spot for an ambush.
Cross the bridge quickly, and watch for smokers. You don't have to enter the house at the other end, but you should – it always has ammo, and usually has explosives. Head back to the path and stay on it. Stay clear of the fence, it's possible to get pulled down. Stay to the middle of the path all the way to the safe room door and you should be able to avoid any unpleasant surprises.
Exit the safe room quickly so you can check the nearby rooms for pills and explosives. Waste no time heading down the stairs and into the first room on the left – this room sometimes has advanced weapons and the side rooms will have a molotov or two. Head out on to the catwalk, but move quickly – competent smokers will try and pull you off, which is unpleasant, if not devastating. You can also drop directly down to check the rooms below for consumables, but you are probably better off just moving to the emergency door. Grab ammo, set up some gas cans and propane tanks outside the door, and fire a few bullets through the wall just in case. (Note: You may encounter a tank during or after the crescendo event, so you might want to hang on to one of those gas cans if you don't have any molotovs).
Open the door, and an alarm will sound, triggering the horde. The safest place to be is in one of the corners of the room, stacked up or spread out. The infected will come through both doors and a vent in the ceiling, but it shouldn't be too much of a problem with the ammo you have available. Head through the emergency door and up the stairs immediately to your right. Stay on the catwalk and stay clear of the edges and windows – again, you don't want to be smoker bait.
After you pass in between the two rooms with machinery, shoot out a window and exit on to the roof. You need to get through the tunnels – you can stay on the ground or stick to the tops of the trains. They both take you the same place so don't worry overmuch about it. It's easy to get swarmed in the cramped spaces, but if anyone on your team picked up an auto-shotgun, you should do fine in the tunnels. Stay close and move quickly and you'll get to the safe room in no time.