Most times I copy a templates HTML and replace the menu w/ {menu}, content w/ {content}, sidebars or other content areas w/ {content block="morecontent"} morecontent can be any thing you want, it shows up when you edit a page as Morecontent: edit area... don't forget to make sure it's set to 'active'...
Then copy the CSS into another/new style sheet and attach it to the new template, Layout > Templates click CSS icon on the right, dropdown choose CSS...
1) where you put your CMSMS folders/files depends on your hosts structures, when I FTP to mine it was blank first time no www folders nothing...
2) unless you are doing some kind of linking from the original site you would make the whole site using CMSMS and then you can give them just the pages they should edit, using Users & Groups...
3) pages have a template choice drop down where you tell it which template to use so it's the page not the template they have to work with...
4) The changes happen on the server it's not like dreamweaver or such...
5) teaching the client depends on if they are going to make template/look changes or just content/add pages changes, if you are the only one who is going to be doing template/style/look changes you don't need to teach them how...
just tell them , login go to Content > Pages choose/click name of page change content like using word click submit and it shows up in the front end/website instantly...
Feel free to ask anything you need help with, that's what were here for...
Link below for CMSMS demo site....