How and what cost to show for shares sold that date back to decades in a itr

Do you mean how CG would be calculated? If yes, here it is:

• Net sales consideration
• (-) Cost (or indexed) of acquisition [whichever applicable]
• (-) Expenses on transfer
• Balance is the CG

Hope this is what you meant to know.

Why should the cost of shares be indexed..that is totally wrong as i said the shares are listed and STT paid so there are no taxable capital gains U/s 10(39). I want to know what amount can be shown as the cost of these shares in the ITR as the shares were purchased in physical form more than decades ago..

Indexation does not apply here

Why can't it be indexed? As I understand, there is no restriction of indexation in case of listed shares. However, there is an option to compute CG without indexation, in which case tax is payable @ 10% (instead of 20%). But if the transaction is covered by STT then CG is exempt u/s 10(38) and if not covered by STT, then you can either opt for 10% tax (without indexation) or 20% tax (with indexation). What am I missing here?