You can setup the funnel by following the below instructions:
You can find the product IDs in the "Products" > "All products" menu, when you hover on any of them in the list.
For example, setup you funnel like that:
And that's how your YES link would look like on your custom upsell page:
<a href="[wc_1cu_yes pid=1]">Add Product #1 to Cart</a>
<a href="[wc_1cu_yes pid=2] ">Add Product #2 to Cart </a>
<a href="[wc_1cu_yes pid=3] ">Add Product #3 to Cart </a>
<a href="[wc_1cu_no]">No, thank you</a>