Press F3 on the menu/intro screen to open the level editor.
Use the scroll view or the buttons on the right to select a tool/object.
You can also press Q to quickly choose the Select tool, and then Q again to switch to the previous tool/object again.
You can pan the view by holding down the middle mouse button (scroll wheel) and dragging. Press the icon on the left of the run button to reset the view after that.
Note that the game currently only supports single-screen levels (no camera movement), so you rarely actually want to place objects outside of the default view (sometimes it's useful, though)
Select tool (mouse cursor icon):
Click things to select them. You can also box select by dragging. Hold Ctrl to add to or remove from current selection (instead of replacing it).
You can drag the selected object(s) to move it/them.
The weird white box on every object is the object's hitbox as far as the level editor is concerned, i.e. that's the part you need to click inside to select them.
You can press Q to quickly toggle between the Select tool and the last used Object tool.
Object tools:
Click to place an instance of the object. You can hold down the mouse button to "draw" objects (they'll never overlap other objects of the same type)
Hold Shift to make a rectangle or line: First click where you want one corner (without holding shift), then hold shift and click where you want the opposite corner.
Connections:
Doors and pressure plates have connections. Simply drag the little circle from the pressure plate to the door you want it to open. I think this works now.
Saving:
Saving is a little weird because of GameMaker's sandboxing/file system limitations. Press Ctrl+S to save to the sandbox area (%appdata% on Windows, local storage on HTML5). Ctrl+Shift+S for Save As and Ctrl+L to load work as well. You'll have to type the filename manually (but you can skip the .chrablvl extension if you want).
Press Ctrl+Alt+S or the "Export" icon (next to the play icon, read the tooltips) to export the level to the regular file system. A standard file selection dialog will open. You can import levels from the standard file system in the same way, with Ctrl+Alt+L or the Import icon. When you do, you'll probably then want to Ctrl+S to save it to the sandbox for easier access from the level editor.
Playing:
Press F5 or click the run button to test the level. Press Esc when in game to return to the editor.
.chrablvl-files are plain-text files. Sometimes you might want to edit them by hand (especially if the editor messes up somehow).