Draw lines (rows of 'nails') by clicking twice.
Draw a second line while another one is selected to create a 'fan'.
select mode
Select lines by their endpoints and fans by clicking on individual 'strings'.
If multiple endpoints overlap, cycle through them by clicking multiple times.
Deselect by clicking somewhere empty or pressing esc.
Line ends can be dragged around.
layer up/down
Make the selected fan be drawn above or below other fans.