The classic pattern is to sit in a circle as each member describes their great dream. This is constructively criticized by other members for design flaws. No invalidation is allowed and most people are positive because they have their own dreams too.
Look closely at the first picture and you will see the window is painted and not real! Both photographs were taken in the Bath area.Once every member is familiar with the plans of the others they apply their collective wisdom to the resolution of problems. Ideally the group should contain people of different kinds of intelligence and come from different areas of life.
Napoleon Hill (who came up with the term) believed that something supernatural also takes place- something like 'the secret' that causes good things to flow by force of will. We may take this or leave it- the technique works either way.
My meetup group (the London Flaneur, Mastermind and Social group) which is London based will offer monthly mastermind groups and I hope to see some of you there.