The OP asks for the basics. Here they are;
1. Use the wiring size recommendations that Stu Jackson points toward.
2. Follow the ABYC Color code.
3. Make a working
Here is the ABYC Color Code (a lot of this stuff does not apply to a sailboat);
In addition, you ask for how to do the engine / alternator circuit.
Here is my current
wiring diagram, for my basic charging circuit for my Universal M-25 with the Westerbeke Admiral's instrument panel on my Oday 35;
Not shown in the diagram is that I have an IOTA 30A Charger connected to the house battery.
You can see that in a few places I did not follow the ABYC color code. One example is the starting circuit, which uses a white wire with a red stripe on my boat. Part of the reason is that I could not find any marine grade tinned yellow wire with a red stripe, but instead I used the AWG10 White / Red that the previous owner paid big bucks to have installed by a pro.
Another example is that my negative conductors are a mix of yellow and black. This is because I am slowly changing out circuits, as needed, and as my budget allows. I am not doing a wholesale rip & replace (I would rather be sailing).
However, I have documented the color code that I have used, and that makes it acceptable per ABYC.