This is a good question and it's one of the most important things to get right.
The method I use for adding content into a Text / HTML module is as follows:
Write the content in Word, Spell Check
Copy the content and paste into Notepad (This strips the formatting & gets rid of excess HTML that Word creates)
Copy this plain text into the HTML / Text module.
Format the text in the module.
Re: formatting within the module:
For the font design of your text, I would encourage you not to select the font option from the drop down list, but to use the default fonts that you have set in your skin.css file (Setting the fonts is explained in the
how to create a skin tutorial)
To then add a skin.css styled font to your text select the text to format & then select from the drop down box in the Text / HTML module "Styles" (or "Format" if you are using the FCK Editor) you can then select the most appropriate class that you have created.
If you do not select a class, it will use the default .Normal class.
It's important to do this as it minimises the amount of HTML code that is created and therefore keeps your page load times as quick as possible.
If you find things are getting messy in the HTML, it's sometimes best to start with a fresh HTML / Text module and then add the content if you have tried several methods of adding content, eg. Visual Studio etc.
Hope this provides a starting point.