Of the many requirements for an international applicant to DIU which should I do first?

First things first: submit an online application form and pay the application fee, if required, once you have decided to apply. Then– 

If you are seeking gateway admission to DIU, 1) Send us your transcripts, and 2) if you are a non-native speaker of English, establish that you have adequate English proficiency for studies at DIU English Language Proficiency Requirements and Send us your transcripts (gateway)  

If you are seeking either graduate or undergraduate admission status 1) Send us your transcripts; 2) send your transcripts for an official transcript evaluation; and 3) if you are a non-native speaker of English establish that you have adequate English proficiency for studies at DIU. See English Language Proficiency Requirements and Send us your transcripts (graduate) and Send us your transcripts (undergraduate)