Short answer: before redesigning a website, list existing pages, important URLs, content to keep, proof to add, images to prepare, redirects, form, mobile, SEO and checks after launch.
Why a redesign can lose traffic
A redesign is not just visual. If URLs change, titles disappear, old pages are not redirected or useful content is removed, visibility can drop.
Before touching the site
List URLs, traffic pages, titles, meta descriptions, important photos, access to WordPress, hosting, domain, emails, Search Console and analytics if available.