Altough it is very simple to file VAT return online, I am just giving some steps in brief to guide you: -
Step 1: Login the website of the State VAT deptt (e.g, www.dvat.gov.in)
Step 2: Go to dealer login option & log in ths with the TIN & password provided to you by the deptt.
Step 3; Choose file return option in Form 16: choose whether it is original or revised return.
Step4: Now simply a form of VAT return will be opened that need to be filled as per the local state's law & the data available with you for your client.
Step 5: After filling this form for VAT, proceed to file return of CST under Form A.
Step6: In the end, varify the details filled & submit. Save the pdf file or take printout of the return filled.
If you still have any doubts, you may ask the procedure in detail by contacting me.