What are the steps to apply for an over 50 visa in Thailand, and do I need to extend my current stamp first?

The user is currently in Thailand on a 30-day stamp which expires on January 12th. They have received an income verification letter from the British Embassy and seek guidance on applying for the over 50 visa. They need clarification on whether they have sufficient time to apply for the visa with their current stamp or if they must extend their stay for another 30 days before applying.