Type editor at the main menu. A "Custom Levels" button will appear. Select it, then activate the following cheats by entering the corresponding keys:

CHEAT UNCTION - CHEAT CODE
Test play the level, press again to return to editor - SPACE
INSERT a sphere or level component at pointer - INSERT
DELETE the selected sphere or level component - DELETE
Toggle on-screen display - H
Hold to reduce the effect of "+" and "-" buttons by 10 - SHIFT
Hold to increase the effect of "+" and "-" on-screen by 10 - CTRL
Return to the editor while testing level with current changes intact - Home
Toggle grid - G
Alter how many segments the grid is split - <0-9>
Alter how many sections each grid segment is divided by - CTRL+ <0-9>
Snap selected component to nearest grid location - S
Hold while moving sphere to prevent it from overlapping other components. - SHIFT