The scheme is open to all businesses that have VAT turnover of £150,000 or less (excluding VAT). A business must leave the scheme when income in the last twelve months exceeds £230,000. Unless this is due to a one off transaction and income will fall below £191,500 in the following year. If there are reasonable grounds to believe that total income is likely to exceed £230,000 in the next 30 days, then the business must also leave the scheme. The turnover test applies to your anticipated turnover in the following 12 months. Your turnover may be calculated in any reasonable way. But if you have been registered for VAT for at least a year, it would usually be based on the previous 12 months.
You can join the scheme by applying by post, or online (when you register for VAT). HMRC will inform you that your application has been accepted and of the date of commencement. Only after that you can operate the flat rate scheme.
The scheme will clearly be inappropriate if you regularly receive VAT repayments.