76:11 The time to recite Musaf begins immediately after Shacharis and one should not delay saying it past the seventh hour of the day. It is considered inappropriate to delay it past this time but one who does so still fulfills his obligation reciting it then because Musaf may be recited the entire day. (If one recited it before Shacharis, while he ought not have done so, he likewise fulfills his obligation – Rema OC 286:1.)
76:12 Let’s say that a person delayed saying Musaf so that now he must say both Mincha and Musaf. In such a case, he should recite Mincha first, followed by Musaf. This is because Mincha is recited more frequently (i.e., daily), and the rule when we must do two things is that the more common thing is performed before the less common thing. (Again, if he did say Musaf first, he fulfills his obligation – Rema OC 286:4. If it’s close to nightfall and he only has time to say one, he should say Musaf because Mincha can be made up with a compensatory prayer and Musaf can’t – Mishnah Brurah 286:13.) However, when davening with a minyan, one does not do this.