Pick a pen, then drag on the grid to lay a piece. Drag a piece to move it, drag its corner to resize. Drag the spawn dots and the ring.
Piece
Editor key
Saving an arena writes it to the machine hosting the game, so it needs the key. It was generated on this server at data/editor-key.txt the first time it started. Open that file and paste the line here. It is remembered on this device.