1. Go to the exchange. Set quality to "Rare" and click a category of Duty Officer twice to search the whole category of DOFF (like "Duty Officers - Science" or "Duty Officers - Medical"). Tick "Only show usable items" at the bottom; DOFFs of another faction won't work.
You are looking for the cheapest blue DOFFs on the exchange to convert to white DOFFs, earning Recruitment CXP in the process.
Buy a few. Don't fill up all your DOFF slots with these DOFFs because each blue DOFF will be converted into 9 white DOFFs and you'll need room for them.
2. Go to your faction's personnel officer. For Federation captains and Fed-aligned Romulans, this is the Human woman who sits behind Lt. Ferra at the Academy. For KDF captains and KDF-aligned Romulans, this is the Klingon man standing next to the console that gives datachips at the academy (in the little building with the exchange and bank consoles). For all Romulans, this can also be the console near the door in the room with the BOFF trainer and Centurion Velaar at New Romulus Command.
3. Click "Exchange officers", then "Rare duty officer exchange". A DOFF mission UI will pop up and you have to "assign" one of those DOFFs you bought just a moment ago. I use the filters to find them since usually I'll buy a bunch of one career so that I don't accidentally convert a DOFF I like.
**Warning** A DOFF that gets exchanged will be lost forever!
4. Get your rewards for that assignment - 3 uncommon DOFFs! Click "Uncommon duty officer exchange" in the personnel officer UI and assign one of those green DOFFs.
Use the filters to separate the more valuable green DOFFs if you want to sell them (green Research Lab Scientists, Damage Control Engineers, Technicians, and Refugees can often be sold for more than what you paid for the original blue DOFFs). Start the assignment and get your rewards - 3 common DOFFs.
5. Repeat 3 and 4 until you run out of DOFFs or DOFF slots. You'll get 150 Recruitment CXP for every blue DOFF and 50 CXP for each green DOFF.
6. When you have too many DOFFs, donate them to the fleet for 250 or 300 Fleet Credits apiece, sell the more valuable ones on the exchange, or mail them to your alts who need more DOFFs or Fleet Credits.
(Also, you can use the purple DOFFs here that you get from the colonization assignment chains at each of the exploration clusters if you have doubles or triples that you don't need. Each purple DOFF yields 3 blue DOFFs and 300 Recruitment CXP, for a total of 1200 CXP and 27 white DOFFs.)
