Although the slider is a most impressive feature of the Stay theme, getting it working is highly problematic for a lot of beginners.
Well intention forum staff need to appreciate that there is little point in sending these confused and frustrated users to the theme's documentation [ http://theme.wordpress.com/themes/stay/ ] because it does a very poor job of explaining what needs to be done to get the slider working.
The responses to all the following posts do just this - send the person seeking help to the totally inadequate 4 step explanation on the Stay theme's description page:
Below, I have provided a more comprehensive explanation of what is required:
1. In your Dashboard, create a Page that in Page Attributes has as its template the Home template.
2. Go to Settings then Reading.
3. Set your Front page to display a static page that is the page you set in step 1 above to use the Home template.
4. If you have previously set a Header image on your Home page, you must remove it (because the Home page must not contain its own featured image if the slider is to work). Go to Appearance then Header and click Remove Header Image, then Save Changes.
5. Go to each of your other pages and in the bottom right hand corner, set a Featured Image.
Max size is 920 × 383. This means that if the photo you use is more than 383 px in height, WordPress can use it, but will crop it equally, top and bottom. Width does not seem to matter - on my site (http://olddairy.wordpress.com/), I used images 960 px in width.
6. Go to Appearance then Customize then Home Slider.
7. Select up to six featured page images to add to the slider then click Save and Close.
The slider containing each of the secondary page images will now operate on the Home page.
It would be helpful to new users of the Stay theme if this description could be reviewed, improved and moved to the theme's home page [ http://theme.wordpress.com/themes/stay/ ] in place of the current explanation; alternatively, this forum post could be referenced from the theme's home page.