The Level Editor lets you create your own maps. It is found in the middle of the Nexus, next to the Custom Levels computer.
Basic selecting tool. Drag your mouse to select, then move that selection around. You can turn off classes of things in your level to not select them when you drag your mouse, and also lock certain layers for selection.
Select a tile to the left, and (if available) a variation of it to the right. The basic unit of putting tiles down is a 2x2 square block. Scroll with your mousewheel to create bigger and smaller circular blocks of that tile. Shift + scrolling makes for sloped blocks. You can change the layers on which to put the blocks here by clicking the numbers, make layers invisible by clicking the left layer box, and lock a layer from being editing by clicking the right layer box. You can replace tiles by holding CTRL and clicking tiles. You can create rectangles by holding ALT and dragging your mouse.
Choose from props categories in which a bunch of props are contained. You can't resize props, but you can tilt them by scrolling with your mousewheel, and you can flip them by pressing [ or ]. Holding shift makes props align into a grid when placing them. Placing props on layers 1-5 makes them larger (as background props). The prop category "backdrops" are specifically meant for these layers and include various backdrops such as mountains. Note that deleting a prop can only be done while the prop menu is selected, and deletes all props on all layers around where you right click. Because of this, it's advised to lock layers 1-5 if you have props in them, which prevents you from accidentally deleting them while working with props in the foreground.
Entities are things players can interact with, such as dust, spikes, enemies, and checkpoints. Dust and spikes can only be placed on places where there are tiles, and will disappear if the tile is deleted. While having selected dust or spikes, you can scroll your mousewheel to enlarge the area you will place dust or spikes.
Enemies can be placed anywhere and follow a path if you set it by holding shift and then selecting where they should go. They reverse the path when they get to a dead end, then repeat it when they finish. Note that some enemies, such as the maids or hedgehogs, can be placed on ceilings and walls by placing them next to one.
There are two kinds of checkpoints: a blue flag, which is a checkpoint where you respawn when you die, and a red flag which sets the ending criteria for a level. When you put down a blue flag, you can hold shift to select the area that the player needs to go through to respawn at the flag. When you put down a red flag, you can hold shift and click on enemies to set them as ending criterium. If you set any enemies at an ending flag, the level will end when the player comes into range of the flag.
You can create death zones by clicking the skull entity, and then dragging the mouse over the area you want to make a death zone.
The player 1 through 4 entities are for spawning players at the start of a level (a single player level only needs a player 1 spawn point). There can't be more than one spawn point for each player in a map.
Create a trigger by selecting the kind of trigger you want to the right (ambience, fog, text, or music), and then clicking a spot. You can edit a trigger by clicking the dot in the middle of it, and resize it by dragging the edges of it.
Ambience changes the ambient noises in your level. You can slide the bars around to create more or less of a particular noise.
Fog changes the way your level is coloured. You can change the stars in the far background at the top, and you can change the hue of each layer by clicking the colours to the left and changing it. You can change the intensity of each colour per layer by sliding the bar next to it - sliding it to 100% will make the layer uniformly that colour. The bottom slider changes how fast the colour changes happen after triggering the trigger. You can save particular fog settings by clicking "Add", and if you click "Use" while having selected a fog trigger, it will insert them.
Music lets you pick the music that triggers when you go through the selected area. You can change the time it takes for the music to set in and the volume, as well as the song.
Text lets you put text in a certain area. Click the text box and press enter to edit.