Governance by announcements.

How to create a petrol/diesel-free nation in a functioning democracy?

Announce a policy intention and call for suggestions and debate.
Appoint a team of professionals to study and create a roadmap. This will include by what date the said energy sources are to be stopped, and so from what dates should the sale of equipment that uses such sources stop, what alternatives could be made available either in buybacks or tax incentives or subsidies to the owners and manufacturers of petrol/diesel equipment at that point, how and who will recycle the obsolete equipment, how would the new energy sources (including the upstream generation of such energy) be extracted, transported, stored, and distributed, what are the short, medium, and long term repercussions of these, and what would the entire exercise cost in terms of time, money, government support, and personnel, among other questions and answers and options.
Use your well-oiled PR machine to get public feedback and buy-in on the objective and the roadmap.
Take into confidence, the opposition, environmentalists, experts, scientists, and industry representatives and discuss the proposal and timelines. Modify and negotiate, cajole and threaten, incentivise and punish, and get everyone (or almost everyone) on board. Let this be a public exercise.
Create a bill. Debate. Modify. Build consensus. Pass it into law. On live TV.
Appoint a committee of professionals to implement this, peopled by India hands as well as international experts headed by a well-known and respected name, operating under Parliamentary oversight.
Execute like hell.
Take regular audits to see if it’s working, and if things are as per plan. Have an open mind. Create a feedback loop to avoid repeating mistakes.
Do not rush to take credit.
Do all this in the full glare of media attention and the public eye.

How it is done in today’s India: