PermaLink Redirects – .htaccess needed?

Hello,

I’ve recently received a large number of soft 404 errors, and I believe this is mainly due me changing my permalink structure about several months ago. I thought this would only affect a handful of links but in the last day or so, I’ve gained over a hundred, and it’s become tiresome to place each one through the Redirection plugin.

Is there anyway that I can redirect all posts with the old permalink structure to the new one – potentially via the .htaccess method? I have a basic knowledge of it, but could edit it myself if I had a step-by-step process of what and where to include it?

The old structure:

http://www.tomorrows-news.com/year/month/post/

The new structure:

http://www.tomorrows-news.com/category/year/post/

If you hadn’t already guessed it, my website is:

Hope someone can help?

Many thanks in advance.

Ed.