Map every URL before you start
Crawl the old site and export every indexable URL, with traffic and backlinks. Each one needs a home on the new site, or a deliberate, documented reason not to.
Redirect one to one
Point each old URL to its closest new equivalent with a permanent redirect. Avoid sending everything to the homepage; search engines treat that as a lost page.
Test on staging, then watch closely
Run the redirect map against staging before launch, keep the old sitemap available briefly, and monitor Search Console daily for the first few weeks. Small issues caught early stay small.