- Add WP_ALLOW_MULTISITE to config/application.php
- Start the multisite install process in the admin normally.
- Copy the generated .htaccess values into your htaccess files
- Add the required multisite constants to development.php, staging.php or production.php, taking care to remove references to subdirectories. At least DOMAIN_CURRENT_SITE should probably be defined as an environment variable in your .env file.
- IMPORTANT: Check the home and siteurl values in wp_options and wp_sitemeta, removing any reference to subdirectories (yoursite.com/wp should be changed to yoursite.com). This is because the .htaccess rules take care of hiding the /wp in URL's. Make sure there are no trailing slashes, otherwise stuff will break.
- Read through point 5 again and verify you did it properly.
- Login to your site at yoursite.com/wp-login.php (NOT yoursite.com/wp/wp-login.php)
- Remove the silly '/blog' from your permalink structure and save.