Alright, this is a simple tutorial on how to edit your layout.
Check what kind of layout it is, be it Flexible Squares or Smooth Sailing etc. Also check which layout system it uses (!important), S1 or S2. S1 is the OLD LJ layout system and S2 is the new one.
If you're using S1, on the "Select Journal Style" page, there should be a link at the bottom of the sidebar (under "Developer's Area") that will lead you to switch to S2. Don't worry, you can switch between S1 and S2 any time.
Go to "Select Journal Style" and choose the journal style of what ever your new layout is. Choose ANY of the layouts, it really doesn't matter.
Search for the style in the box at the top right hand corner. Just by typing the first word/letters, the full style should appear.
Eg. type "Smooth" and Smooth Sailing will immediately appear.
And then just take your pick!
Go to "Customize Journal Style" and then to "Custom CSS". See the big box there? Insert your coding there.
If the layout looks screwed when you preview it, make sure that "Use layout's spreadsheet(s)" is at "NO", "Use layout's stylesheet(s) when including custom external stylesheet" is "NO" and the last one is "YES".
Done! You may now customize the other features of your skin, for example the sidebar.
On the same "Customize Journal Style" page, there will be a link to "Sidebar". There, you can choose to display tags, page summary etc.
For a more comprehensive tutorial, head to any layout community such as premade_ljsand feel free to ask me!