I am using beta theme 1.0.5 version  and i am modify many theme, osclass, plugins core files.

Now i cant update latest versions due to lost old modification.

If i can setup proper child theme and make modification also same time i am updating main theme version. here

i want -

   1) keep my modification in child theme ( theme php , css , js files )
   2) new version theme work properly
 

 Kindly any detail guidelines for setup child theme...?

 pls help me.

I was testing child themes year ago and it was not functional, there was some issue especially with functions.php
You may get more luck