How you perform the act of formatting text depends largely on the editor you choose to use. Nevertheless, the process is largely the same the rich text editor and VisualEditor — both of which have pictographic editing tools.
Though some individual wikis have added these editing icons to source mode, formatting in source is generally quite different, requiring a knowledge of wikitext that is explained elsewhere.
Formatting with either of the visual editors
Once you click the edit button and enter the editor, you will see this toolbar on top of the page.
Highlight the text you wish to modify with your cursor, and then choose:
to make the text bold.
to make the text italic.
to underline the text.
to strike-through the text.
Beyond the basics
As you can see from the above images, there are many more buttons than we've mentioned. You can perform all sorts of formatting, much of which has been demonstrated on this page. Since formatting in Fandom's visual editors is largely a matter of highlighting text and pushing one of the buttons, take some time to explore the interfaces. You'll easily learn how to perform formatting much more advanced than italics and bolding.