1)    One who prays a supplementary prayer, should bring it as close as possible to the obligatory prayer that he is praying. If he did not start it immediately there are some halachic authorities who maintain that he may no longer pray the supplementary prayer. The ruling is that if one did not start the supplementary prayer immediately, one should stipulate before starting that if this prayer is not valid as a supplementary prayer, it should be a voluntary prayer instead. (Shulchan Aruch and Mishne Berurah 108)

2)    One only has the opportunity to pray a supplementary prayer in the prayer which comes directly after the one which he missed out, and not in the subsequent one. It follows that if one missed the morning prayer one may supplement it during the afternoon services, but if one forgot both the morning and afternoon services one may supplement during the evening services only the afternoon services, but not the morning services which no longer have any rectification. (ibid)