<p>There are no hard-and-fast rules for essay formatting. Legibility is an important consideration. Double-spacing or 1.5-spacing is a good idea; make sure it's in a standard, reasonably-sized font as well (Arial, Times New Roman, or something similar, 12-point).</p>
<p>The print-preview version of the Common App says to put full name, date of birth, and name of secondary school on each sheet of your essay. This isn't in the online instructions, but you may want to create a header with that information, just to be safe. Papers can get scrambled and if you have a popular name, they may need extra information to figure out whose essay it is.</p>
<p>According to the Common App website, you can submit it as a .doc, .wpd, .rtf, .xls, .pdf, or .txt file (but not .docx, if you have the new version of Word).</p>