Click grey arrow to start Click while in the editor to set the spawn point. Press "d" place blocks. Press "s" to delete blocks. Press the right and left arrow keys while in the editor to switch blocks. Press space to advance from the editor to player mode and vice versa. Arrow keys control the player in player mode.
THIS IS A WORK IN PROGRESS; WHAT I HAVE NOW IS THE BARE MINIMUM. I will be adding new features and refining the user experience soon (because there is much to be desired right now ^-^). Improvement suggestions are welcome, however. This is a project I am actively working on and I will get back to you about your ideas.