Re: Supererogatory prayers
As per what i know its either way.if you go early in masjid and has time for salah ,offer a qaza instead of any nafl other wise offer them after fardh. and then in weekends you can offer them any other time suiting you like between fajr and dhuhar you can offer say one days complete qaza.
The issue is when there is only little time for one salah , in that case you must offer current fardh first.
Just make a time table /chart of your qaza salat and then start offering.
PS: try offering as much qaza as possible like offering qaza instead of Sunnat ghair mukkada and nafl.