LED resistor calculator :
A 680 ohms resistor should do the job, thats blue,grey,brown,gold 1/4 watts carbon resistor. The colour gold is the +-5% tolerance. The colour coding will be different for metal oxide type...google it!!! Connect the resistor to the +12V and the anode of the LED, the LED cathode (flat side) goes to 0V.
Always connect the fuse just after you tap into the main supply...In your case its between the battery and the switches.
You can connect all the +V of the switches into a common wire then into the battery through a single fuse. Just make sure this wire can take all the current you wanna drain.
If you need +V supplies with different fuses then of course you need to run separate wire before feeding them to the battery throught different fuse rating.
Fuse rating...just check the total current drain of the devices.
I hope thats clear enough