When making levels for a game it is important to document your ideas, this way if your on a team everyone is on the same page and if your working by yourself you have something to work off of. A one page level design document can make the process of creating a level much easier. The methods that I am going to discuss have worked well for me but like anything in agile game development none of these rules are set in stone.
The first thing I like to do is make an illustration of the level this can be done in any program you prefer such as: Photoshop, Adobe Illustrator, or even Google Docs. If you have created a concept sketch digitizing that will make the process easier. It’s important to remember that if your level is 3D then you are making a 2D drawing to represent it, this can lead to confusion. In the cases where I have intricate 3D levels I like to have a top down drawing and a side drawing, this will help anyone else who sees the document understand the space. Once the drawing of the level is complete I like to draw arrows to the most important parts of the level and have boxes of text explaining any special mechanics that happen in those areas. If you document is for a single player level it is important to make a golden path. The golden path is the most ideal path a player can take to complete a level. Multiplayer levels do not have golden paths because where the player needs to go changes as the match progresses. The last thing you need once your drawing is complete is to make a key to show what certain symbols represent.
Another important thing you need to have in a level design document is a asset list, this is a list of every art and sound asset that the level will need to be completed. This is very helpful if you are working with artists and other designers. The list will also give you an idea about how long it will take to complete making the level.
Level design documentation is very important if you are working on a team but it can also help you if your working by yourself. When making a level design document remember that no matter how good of a level idea you have it does not matter if your document is not readable.