Hi Zach0.
Well, about this kind of pronunciation it's quite complicated to explain but easy to spot the difference if you hear both words several times.
About "mitsu" you just have to pronounce it as you read it. It means saying "mi" as in "you and ME" and "tsu" as in "TSUnami".
About "mittsu" it will sound more like if there is a pause between both syllabes. If you want to say it properly, just try to pronounce quickly the "t" in "mit" then "tsu". The "t" in"mit" must be practically inaudible and as it's quite difficult to pronounce it quickly you should instinctively create a short pause between both syllabes. Just try to hear both words again and again and practice the pronunciation just after. It should be fine ;)
Sorry for such a large paragraph ^^ This technique is quite the same used for the double "r" in spanish and it worked for me. I hope it will for you as well. Have a nice day and gambaru !