[BeccaPrice]

I'm very new at using Calibre. If I have an html file that I want to convert to epub or mobi, and have a CSS file, how can I use Calibre to attach that CSS to my text file? Or do I need to use something like Sigil to do the attachment, and then use Calibre to convert my file to mobi? I'm trying to make files that can be read on either Kindle or epub readers, on anything from a smartphone to an iPad, so formatting has to be kept simple and to a minimum.