Yes i will Guide you. Your Problem is very simple. You just want to create CSS file right? so its very Easy.
What ever the tools you have to use notepad++, notepad, sublime etc. Just open a new file and write your CSS into those file and
Give Extension .css exa: abc.css
and after created those file you need to linkup with your html code. you just write
<rel="stylesheet" type="text/css" href="abc.css">