Praise be to Allah.
Shaykh Muhammad ibn Saalih al-‘Uthaymeen (may Allah have mercy on him) was asked this question and he replied as follows:
What we advise him to do is to begin with al-Baqarah, unless he has memorised from al-Mufassal [the soorahs beginning with Qaaf, until the end of the Qur’an – al-Haafiz Ibn Hajar], because for some people it is easier to memorise from al-Mufassal because the soorahs and verses are shorter, and they hear them a great deal from the imams in the mosques. So if that is easier for him, let him begin with that which is easier. We also advise him to pay more attention to what he has already memorised than paying attention to memorising a great deal, because paying attention to what is already there takes precedence over paying attention to what is not there.