Remove the main navigation
This will reduce the likelihood of them navigating away to another page of the site and abandoning their cart.
For a good example to illustrate this best practice, take a look at how Shopify does it (see below). The header and footer are removed, but they keep a couple of ways to go back to the cart / bag (the breadcrumbs and a back link, highlighted in red).
Keep minimal links
Keep minimal links in the footer, such as your FAQ, shipping & return policies, and contact info.
That way, it focuses the shopper on the checkout and avoids distractions, while still allowing them to keep shopping if they’re not ready to buy.