amp and door speaker wires

ok so I have most of it figured out on how to install a 4-channel amp to my door speakers and into my head unit.  the only thing i am confused about is how to run the speaker wires.

1.  biggest question, do the speakers wire into both the head unit and the amplifier? 

2. if so how?

3.  how do the wires connect into the speakers IF you have to wire them into both the head unit and the amp?

also a little side note/question, how do i figure out what size of fuses do i need (which would probably include how do i figure out how many amps i have/need to run through the amplifier)?

detailed explanation would be great, or if you know a good website that would be fantastic!

Thank you, all who respond.  I am thankful for the time you take to reply and help me out.

first check out bcae1

Secondly, it really sounds like you may be in over your head here, if you do not know that the head units power is no longer used when using an external amp you may be in for some problems. You may how ever, catch all four speaker leads at the deck, using the facotry wiring from there. This will make life much easier.

Fusing is based on wire size.