To simply run lepton-netlist and get the output of a backend, use the
option -g backend-name. For example:
lepton-netlist -g custom-backend schematic1.sch ...
By default, lepton-netlist outputs the result to a file named
output.net.
Output file name may be changed by the option -o filename. If the option argument is - (dash), the output is redirected to the standard output of your terminal.
To output your netlist to, say, schematic.net, type:
lepton-netlist -g custom-backend -o schematic.net schematic1.sch ...
To display the output of the backend in your terminal, type:
lepton-netlist -g custom-backend -o - schematic1.sch ...