You can choose 'Create StyleSheet' from the Insert/StyleSheets pull-down menu.
This will bring up a dialog with a few basic HTML Tags built in and allows you to add tags you need to define in your stylesheet. For instance, if you wanted to have a certain font, or font color in your Table Cells, you could click the 'Add Tag' button to add 'TD' to the pull-down list. Likewise, you can also remove tags from the list with the 'Remove' button.
To add a 'style' to a tag, click the check box to the left of the option you want to add. This will enable the pull-down list to the right. Then, you can choose from the pull-down list. On each tab (FONT, ALIGN, COLOR), you make your choices and then click the Apply Button. You will see your changes appear in the text box on the right side of the StyleSheet Dialog box. If you want to change an option you have already applied to this text box, you must delete it manually, then go back to your StyleSheet choices to add it back in again.
Once you have finished creating the stylesheet, you can choose to embed it in the existing active HTML/ASP document, or save it as a separate stylesheet. If you choose to save it as a separate stylesheet, it will also, automatically add a link to the saved stylesheet in the existing active HTML/ASP document.