Keep in mind that if you keep the mouse button down, you'll continue the currently initiated action, e.g. drawing on the grid with the selected piece, or picking up a piece from the grid or the palette.
•On the right side of the screen is a set of self-explanatory text buttons.
To erase some pieces from the grid just use the "Select Eraser" option.
Using the Cut and Paste options (from the Edit menu) will let you easily create "sets" of similar levels, by copying and modifying a "basic" level.
•Always test the level you've just created before actually playing it!
The level is completed when the dots have all been actually eaten. If you leave a dot impossible to reach (surrounded by walls in every direction, for example) you won't be able to finish that level. This definitely ain't nice if you're playing your best-record-match!