How would I go about making an outdoor environment in dungeons and dragons? I would like to know what the rooms and hallways would be. I would really appreciate it if someone would help me. I'm really interested in making a forest.
Go wander outside your house or apartment or whatever and look for a wooded area. Write down a description of it. You've created an environment, then when you read that to your players, they are in it.
Does it have corridors or rooms? No. It might have paths, and clearings that might make good places to have something happen, but the outdoors largely lack the constraints of a dungeon environment.
Do you need a large scale map? If you do, You could map a wilderness area and place some encounters. The first edition DMG has a method for randomly generating type of terrain in each map hex. Consider reading it, or running one of the old 1e adventures to get an idea of how the game works in the wilderness. Or use a real wilderness map from a local national park (or from Google).
Do you need a tactical map? Because often you don't, you know, the description and your players' imagination is more than sufficient. You can make maps of such areas easily, here, check out this random wilderness map generator.
(Note that this question is tagged 2e, so the 4e/late 3.5e concerns about having tactical maps for everything are not very relevant - in 2e we used minis and tac-maps pretty rarely.)
First off, I'd strongly suggest you read through the Dungeon Masters Guide (DMG). Here's a bit of specific guidance for your question.
When building an adventure locale for a game of D&D, you have to remember that this is a place in your world. A collection of corridors and rooms in one sort of place you could imagine for your players to explore. Imagine others. What is is like to walk through a forest? What sort of places might they run into? Where do the paths run, what is there to find where paths cross, what is there off in the forest if the players leave the path? What sort of noises, sights, and discoveries will they notice, and which ones will make them say, "We go investigate closer!"
This is what making a wilderness adventure is about. Create interesting places, usually using a map to help you keep straight where everything is in relation to other stuff, and ask your players what they do. Describe what they're near, and find out what they do next. Creating a wilderness area is a creative exercise, and isn't just a matter of snapping together a few corridors and rooms with some monsters in it—you're creating a place that, hopefully, your players can believe in. You might find the answers to this RPG.SE question about how to start creating a setting for your game to be useful.
Especially in a wilderness, using a random encounter table (also called "wandering monster" tables) will help you decide what happens next when the players wander into a blank area of your forest map. The Monster Manual (AD&D 1st ed) and Monstrous Manual or Monstrous Compendium (AD&D 2nd ed) contains random encounter tables. The Dungeon Master's Guide has guidance on how often and how to roll for random encounters (p. 174 in the 1st edition; p. 96 in 2nd edition).
For a particular encounter outside in a forest, I just scatter trees and try to make interesting tactical locations that the monsters can use to their advantage.
When you are building an adventure, you can build it like a dungeon, even in a forest. Consider a dungeon as just a series of encounters that you can reach by traveling through passages. The same thing can happen in a forest. Don't map out the whole forest. Just describe travel through it. Think of the "dungeon" as just a series of decisions that the party can make. These decisions lead to the various encounters, just like a hallway in a dungeon leads to a room with monsters in it.
When the party hits an encounter location, then you pull out a map, arrange the party and monsters on it, and have at it.
As an example, the party may find a caravan that has been ransacked by bandits. They know that the caravan came from the north, and the bandits are rumored to be living deeper in the eastern forest. So they have a choice here (I made it directional, but it needn't be). It's like going forward or to the right in a dungeon. Either choice can lead to an encounter, just like in your dungeon.
I recommend using mind maps to design your outdoor environment. Unlike a dungeon, the rooms are not connected by hallways and doors. The party may need to travel a mile down a path to get from one area to another. You can categorize your environment into rooms, doors, in-betweens, and landmarks.
Rooms are the main areas where events take place in your adventure. A room might be a forest clearing, a river crossing, or a fallen tree for example. They are connected by doors. The players take paths to get from one room to another.
Write down the name of each room and draw a circle around each. You should place them in relation to each other, so that a room to the north is higher on the paper than a room to the south. It doesn't need to be exact.
Write a description for each room so that every time the PCs come back to it, you can read the same description to them. The description should include obstacles in directions you don't want the group to travel, for example a cliff or tangles of thorny vines that make travel difficult.
Doors are placed on the mind map by drawing lines between rooms to connect them. A door may be a road, hunting trail, or a river. When the party goes through a door, essentially they are exiting the room and traveling down the path to the next room. Each door should have a quick description including how long it takes to travel, and what type of terrain it is.
Outdoor environments are obviously less restrictive than indoor. The party may decide to go over the obstacles you place, and enter territory that isn't a room on your map. To handle this, you create in-betweens. In-betweens are just like rooms, except they aren't placed on the map ahead of time. When the group travels through an obstacle to an area that isn't on your map, you grab one of your in-betweens, to use as the room they enter. You should draw up a handful of in-betweens for the different types of areas they may encounter. For example, you could make an in-between called "large tree" and drop it into any forest location between rooms. When your group discovers it, mark it on the map and draw a line representing the door connecting it to the room they came from.
Landmarks can be seen from almost anywhere in the wilderness, like mountains off in the distance. These should always be in the same direction, and will give your players a sense of direction in the wilderness.
For more information, check out this great article
The way it normally works is like this. A path in the forest, is your 'hallway'. The Trees, becomes pillars, and many trees grouped together extremely tightly could become 'walls'. Very often, there will be a glade of some sort, which is the right spot for a camp site or just a 'room' to have an encounter in.
This is assuming that you are trying to convert an underground dungeon into forest. Obviously, you want to give your players as much freedom as possible, but mechanically, this is how you can do the conversion.
Depending on how you define "dungeon", I'd suggest a village or town which has various encounters in/around the houses. You would have to control access to this town, so maybe put it on a small island, but then you would have to explain WHY the island is cut off from the shore.
It isn't clear to me if you are approaching this from a tactical (miniatures, etc) perspective or from a story / scenario perspective.
Lets say you are building an elven forest, one where the native elves do everything in the forest. Make a list of their needs for day to day living, ie homes, food sources, recreation, defense, and the like.
Also, ask yourself a few questions like...
-why did they settle here? -how integrated is magic in their lives? -what other territories does it brush up against?
Now consider what could happen if they have lived their hundreds of years, how they could scuplt the terrain, modify streams and rivers, plant/replant/transplant plants, leverage native or imported wildlife, etc.
What might be a system of rooms or corridors in a dungeon become regions of the forest, yet, these are wall-less regions and so need appropriate, protective barriers.