If you're a Playstation Plus member, you can now download the Dark Souls 3 stress test client from PSN. However that's not all you need in order to play the game, additionally you will also need to be provided with a code from Bandai Namco as per their sign up page. Unfortunately sign ups closed on October 4th, and Namco still needs to select players from the sign ups.
If you are one of the fortunate few that do end up with a stress test code, the stress test periods are coming up quickly:
- Friday, October 16th, 2015 at 8:00 p.m. to 11:00 p.m. CEST
- Saturday, October 17th, 2015 at 8:00 p.m. to 11:00 p.m. CEST
- Sunday, October 18th, 2015 at 8:00 p.m. to 11:00 p.m. CEST
Currently the stress test period is scheduled for just the Playstation 4 despite the game also coming out on PC and Xbox One. It sounds like From Software is mainly testing balancing and internal gameplay mechanics with the game for right now though.