Preparation of Sodium Peroxoborate
Sodium peroxoborate can be prepare using two methods. 


Electrolysis of sodium borate with some sodium carbonate
Oxidation of sodium meta borate using $\ce{H2O2}$


Heating borax strongly will give a mixture of sodium meta borate and boric anhydride.

If you can remove boric anhydride from the transparent glass like mixture, you can then add $\ce{H2O2}$

$$\ce{2NaBO2 + 2H2O2 + 6H2O -&gt; Na2[(OH)2B(O - O)2B(OH)2].6H2O}$$