Start with checking all battery and starter cables and connections that they are physically clean and tight.
From there you will need a multimeter to check voltage available at the batteries and the starter to see what is missing. Once you find out whats missing you can isolate the cranking circuit from the ignition circuit and get some kind of direction. You tube basic starter checks should get you on the right path. Voltage dropping cables and connections is the only way to know the circuit has integrity.
Understand how it works and why before you dig in. This will make things easier once you start measuring.