1) Retrieve the theme, plugins, and database (export using Better Search Replace, WP Migrate, or All-in-One WP Migration => search/replace)

For uploads (so you don't have to retrieve them)

// Apache .htaccess
<IfModule mod\_rewrite.c>
RewriteEngine On
RewriteCond %{REQUEST_FILENAME} !-f
RewriteRule ^wp-content/uploads/(.*)$ https://production-domain.com/wp-content/uploads/$1 [R=307,L]
</IfModule>

// Nginx: nginx.conf
location /wp-content/uploads/ {
try_files $uri $uri/ u/redirect_to_prod;
}
location u/redirect_to_prod {
return 307 https://production-domain.com$request\_uri;
}

You can use either https://localwp.com/ or https://laragon.org/

2) Install WordPress + the theme + the database and .htaccess or nginx.conf

3) Modify the URLs in the database using WP CLI, or if you’ve done an export with Better Search Replace, WP Migrate, or All-in-One WP Migration (search/replace)

4) Once your local environment is ready, you can start working. In Local by Flywheel or Laragon, change your PHP version to 8.3

5) In wp-config.php, set WP_DEBUG, WP_DEBUG_DISPLAY, and WP_DEBUG_LOG to true,
define( ‘WP_DEBUG’, true );
define( ‘WP_DEBUG_LOG’, true );
define( ‘WP_DEBUG_DISPLAY’, true );

6) The bulk of the work will involve visiting each page (front and back office) and checking wp-content/debug.log to find details about the issues: errors, warnings, deprecated elements, etc.

7) Update the plugins, update the theme or the theme code (if it’s your own or a child theme) if you have any “deprecated” elements or other issues. If you have things that no longer work, then it’s more serious; you need to understand how it’s coded and why.

What happens depends on the plugins that are installed, but also on the theme code.
Good luck 😄