To set a class as a selector in CSS, add a period followed by the class name. In this case, it's .grey-element.

Order the lines to see it in action.

<html>
  <head>
    <link href="style.css" rel="stylesheet">
    <style>
.grey-element {
background-color: lightgrey;
}

</style>
  </head>
<body>
<ol>TO DO:
  <li class="grey-element">Floss</li>
    <li>Feed Cat</li>
    <li class="grey-element">Nap</li>
    <li>Plan holiday</li>
    <li class="grey-element">Order tea</li>
</ol>
</body>
</html>]]>

Make sure to place .grey-element { first, then background-color: lightgrey; and finally }.