There are three steps. You do the first two inside the app, and the third in your theme editor. We don't ask for permission to edit your theme, so that last one has to be you.
1. Choose the products you want compared
Open Products and use Add products. Pick the ones where being cheapest actually matters. There is no advantage to tracking your whole catalogue, and your plan limit applies to what you track, not what you sell.
2. Add the competitor pages
For each product, paste the URL of the page on a competitor's site selling the same thing. You can paste several at once, one per line. For anything more than a few dozen products, use Import instead: export your products, fill the competitor columns in a spreadsheet, and upload it back.
3. Add the block to your product page
In your theme editor, open a product template, click Add block inside the product section, and choose Price comparison under Apps. Save, and the block appears on every product with prices to show.
If you skip this step everything still runs, you just will not see anything on your storefront. The Overview page reminds you until it is done.