It's the same basic concept that you see in the Star Wars original trilogies special edition, but using the original series of Star Trek. Replace all the old Visual effects with modern CG ones. In this case 100% of the exterior shots of the Enterprise and space. Plus the view screen, transporters, and phasers on the interior shots.

Here is what has been done. It's the first act (including the opening credits) of "The doomsday machine." On youtube.

https://www.youtube.com/watch?v=D9XHmj-dPEY

The creators site. http://www.trekenhanced.com/

This might actually make the TOS watchable, if Paramount ever had the brains to do it. Even if was only as a DVD release.