Please be aware that to qualify for the VAT FRS, you should have already received a decision with regards to the suitability of the type of your business you are running as well as the total turnover. HMRC should then advise you of what flat percentage rate you should be using as an alternative to the standard rate.
In general, businesses below a certain VAT turnover threshold, can qualify to use the VAT FRS. Refer to HMRC website to find current maximum turnover.
Any business with a VAT turnover of the stated amount on the HRMC link or less can qualify for the VAT FRS.