Changing Permalinks

Hi.

I’m sorry if this has been posted before.

I’m trying change my permalinks settings so that I get a better URL.

I want to change the setting so that the url comes up like https://jeffturnbull.com/blog/NameOfPost

Reading another post someone suggested checking the Custom Structure to /%postname%/ which I then did.

Is this correct? Because when I did this, if then try to find an old post or page it comes up with an error box to say Oops! This link appears to be broken.

Can someone please let me know what I am doing wrong or missing out?

Many thanks

Jeff