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 Quran – al-Haafiz Ibn Hajar], because for some people it is easier to memorise from al-Mufassal because the soorahs and ayaat are shorter, and they hear them a great deal from the imaams 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.
From: Fataawa al-Shaykh Muhammad ibn Saalih al-‘Uthaymeen, Kitaab al-‘Ilm, p. 212
It was narrated that Abu ‘Abd al-Rahmaan said:
The companions of the Prophet (peace and blessings of Allah be upon him) who used to teach us Quran told us that they would learn ten ayaat, then they would not move on to the next ten ayaat until they had learned the knowledge contained therein and how to act upon it. They said: So we learned the knowledge and how to put it into practice.