Adding a Child's Theme but not Destroying my existing Theme

Hi there,

I need some help.

I am currently using the theme Blaskan – http://the365effect.com/ but I want to start working/learning how to use my custom Theme – Striking by Kaplin

How do I start working on my Striking Theme while at the same time not disrupting my existing site until I am ready to make the big change?

Any help would be very much appreciated for this newbie.
Cheers,
Alan