Click an empty cell to add a node; drag to move; drag the round handle onto another node to connect; click an edge or a zone's margin to select it. Arrow keys move the cell cursor, Enter adds or selects, C connects, Delete removes.
More shortcuts
Marquee-drag empty space to select a range; Shift-click adds to the selection. ⌘C / ⌘X / ⌘V / ⌘D copy, cut, paste, duplicate; ⌘A selects all. Alt+Shift+↓ or → inserts a row or column at the cursor; Alt+Shift+Backspace or Delete removes the cursor's row or column.