How to write program on the stages process

Hello,

On the opportunity, there stages like Develop, Proposal, Close.

I need a formula for this.

We need to add a filed at the develop stage (Top) If Develop  proposal YES/NO?

In Discovery Stage

Only Question on Top should be

                Develop Proposal Y/N

                                If No, then

Text field pops up asking Sales personnel to type in reason for not preparing a proposal then,

Opportunity is closed and notification gets sent to Sales Director saying “XYZ” Opportunity has been closed ( if we can state reason why it was closed in the notification that would be ideal)

                                If yes, then once saved (and all mandatory fields are completed) , the opportunity is moved on to Proposal stage

                Please make the following fields also mandatory in this stage:

                                Customer Budget

                                Decision maker

                                Purchase Timeframe