When enrolling yourself into Making Tax Digital for VAT you should consider the timing very carefully Once signed up, HMRC will expect all future VAT returns (and any current or previous returns outstanding) to be filed through Making Tax Digital software. There are further restrictions if you pay your vat by direct debit as it is currently taking around 15 days for the direct debit to be transferred to the new system.
As an example, if your first full vat quarter, falling on or after 1 April 2019 is for the period 1 April 2019 to 30 June 2019, then, if you have a direct debit, you should sign up on or after 15 May 2019. If you do not have a direct debit then you can sign up from 8 May 2019 i.e. after the VAT return for the previous quarter to 31 March 2019 has been filed.