Refer to the following site to set up the certificate and put the following .htaccess as root to complete the process.
It's a bit jammed.
さくらのサポート情報 【無料SSL】サーバコントロールパネルからの導入手順
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| # BEGIN Force SSL for SAKURA | |
| # RewriteしてもHTTPS環境変数を有効にする | |
| SetEnvIf REDIRECT_HTTPS (.*) HTTPS=$1 | |
| # 常時HTTPS化(HTTPSが無効な場合リダイレクト) | |
| <IfModule mod_rewrite.c> | |
| RewriteEngine on | |
| RewriteCond %{ENV:HTTPS} !on | |
| RewriteCond %{REQUEST_URI} !^/wp-cron.php$ | |
| RewriteRule .* https://%{HTTP_HOST}%{REQUEST_URI} [R=301,L] | |
| </IfModule> | |
| # END Force SSL for SAKURA |