You can choose to copy and paste an entire section into the box, if you only want to use one of the new fonts.
If you'd like to use multiple fonts, however, then you'll need to do this in stages like I did to achieve the Twitter profile name at the beginning of the guide. (Simply repeat step 2, step 3 and step 4 for each portion of the text in a new font.)
Highlight the font you want to use and copy the text.
Paste the highlighted text into your Twitter profile or bio. It should appear as the new font.
In order to achieve the kind of effect you see here, you'll need to copy and paste your Twitter profile fonts in stages.
Check to make sure the changed profile fonts appear the way you intended!