Tashlumin (praying a make up 'Amidah for one that was missed) can only be done during the next prayer. If one did not pray Shahrith, one makes it up during Minha only, by praying the Minha 'Amidah twice. If one forgot Shahrith and Minha, one can only make up the Minha prayer by praying the 'Arbith 'Amidah twice.
Ordinarily, we must pray Musaf before Minha, even though Musaf can, theoretically, be prayed anytime throughout the daytime hours. If one forgot to pray Shahrith and Musaf and remembers at Minha time, what is the best way to proceed?
In such a case, one should pray Minha first, followed by Musaf, since Musaf can be prayed anytime during the day. Shahrith should be made up by praying the Minha 'Amidah again after the Musaf. If one prayed Musaf before Minha, however, one still fulfills one's obligation. It must be reiterated that once the day is over, if one has not yet prayed Musaf, it can no longer be said or made up.
(See Qesher Guddal, 22, Oth 1-3. Ben Ish Hai, 1st year, Parashath Mispatim, Oth 11)
Kesher Gudal Chida, tefillah tefillos, tashlumin, shachrit mincha arvit, shachris minch maariv, shmonei esrei, shmoneh esreh, ben ish chai, parashat mishpatim, musaph