You are in MTD for VAT from your first return, so there is no grace period to grow into. That means MTD-compatible software connected to HMRC before your first VAT quarter ends, not after. The sequence we use is: get the VAT number, set the software up with the correct scheme and registration date, connect it to HMRC, then reconstruct any transactions from the registration date that predate the setup. Do not leave the software until the first return is due — you will be rebuilding a quarter from bank statements under time pressure. It is also worth checking whether you can reclaim VAT on things you bought before registering, which is possible within time limits for goods you still hold and services recently received.