Your voiceover demo should be recorded AFTER you've had plenty of coaching. After several months of training, your coach or coaches will make a determination on whether or not you're ready.
A two-hour workshop on a weekend does not prepare you for a demo. Reading a voiceover book does not prepare you for a demo. Being a member of every single VO group on Facebook does not prepare you for a demo.
One of the biggest mistakes new talents make is thinking they can do it all themselves. Kick that way of thinking to the curb and trust VO pros, talent agents and coaches who have been in this business for decades.
If you're serious about having a chance to succeed in this business, you'll need to take the proper steps and it all starts with lots of coaching upfront.