You can find more information on flat rate scheme on VAT notice 733 and perhaps apply to be on the scheme.
HMRC will notify you in writing if your application is successful. The letter will tell you the date you can start to use the scheme. This will normally be from the start of the VAT period following receipt of your application. If you request an earlier or later start date, HMRC will consider all the facts including the timing of your application and your compliance record. They will not normally allow you to go back and use the scheme for periods for which you have already submitted your VAT returns.