1 Doubt a specific diagram would help much.
2 Basically there is a red "hot" from the battery to the fused switch plate. Off the switch plate comes the hot for each circuit, red. Then from the circuit to a "bus/block" for your grounds. Off this block, in similar fashion, goes a single black ground to the battery ground. So what you end up with is this sequence; battery pos terminal, hot wire, switch, fuse, hot wire, load, black ground wire, neg ground bus/block, ground wire to battery, neg battery terminal. If you didn't use a bus, you would be trying to connect all those ground wires to your battery. Sometimes all the ground wires tie into a single ground wire without the bus. But then you have a big wad of wires tied in together. A buss is better. But you will always have the switch plate. It is the simplest way to get the switch and fuses between the battery and the load.
3. And on my boat, I could not run the "new" hot red wire to the mast under the cabin liner, as the wires were run then the boat put together. They weren't worried about us years later, they only wanted to build fast and cheap. I had to go outside the liner, seeking along cracks, corners etc.
4 if you r not familiar with 12 volt circuits get some help, or order the book from don Casey on sailboat electrics. Once "2" above makes sense to you, then You can do it. But not before that. Then it is Ike a puzzle and you just have to get in there, take it apart, and put it back together.