move vertex
Vertices are draggable.
create vertex
Click on empty space not too close to existing vertices.
select a vertex
Click on existing vertex.
create/erase edge
Select the first vertex. Click on another vertex (different than the selected one) to turn on/off (toggle) the edge between them.
keep the selected vertex
Hold 'SHIFT' to perserve the selected vertex after creating/erasing an edge.
delete vertex
Double click on vertex or drag to the edge for the frame (be careful, there is no UNDO yet!).