Based on your description, I'm not sure you were looking at the right tutorial. The WordPress.comk theme showcase page about Fresh & Clean has the info you want though. Here are the instructions from that page:
Fresh & Clean can display an optional featured slider on the front page that highlights your Sticky Posts that have a Featured Image at least 840 pixels wide. Follow these quick steps to add a post to the featured slider:
- Create a new post (or edit an existing one), and be sure to mark the post as Sticky.
- Add a Featured Image to the post you created in Step 1. The image must be at least 840 pixels wide, or else the post won’t appear in the slider.
- Optional: Create an Excerpt for your post, which will appear in the slider along with the post’s title and Featured Image. If you skip this step, WordPress will automatically generate an Excerpt for you.
- Save the post, and view your site to ensure that it is appearing inside the slider!
<p>Repeat the above steps for each post you’d like to add to the slider.</p>
Here is a link to the page for reference:
Make sure to pay attention to the size requirements for the featured images.