You can create SEO-friendly URLs in Tools > SEO URLs.
What it does is "route" or redirect you and your customers so that they can see your desired URL.
For example, you can change the last part of your URL from /index.php?route=account/guestorder to your desired one, e.g., /guest-order-history, which is SEO-friendly.
Your visitors would see that SEO-friendly URL and you can also use it to create links.
In Tools > SEO URLs, create a new SEO-Friendly URL by typing in 'account/guestorder' in the Route field (we discarded the /index.php?route=) and typing in your desired URL in the URL field, e.g., 'guest-order-history', without the single quotes.
There are many other examples already set in SEO URLs that you can use as a guide when creating your own SEO URLs.
Important: You may have to remove the '/' before index and before your SEO-friendly URL to make the relative path work when creating links for these cases.