This is how I learned them: When you pronounce す your tongue should be lower in your mouth, behind your bottom row of teeth. It should be in the same place as when you say さ or せ. When you pronounce つ your tongue should be touching the ridge behind your upper row of teeth. This is similar to when you say た or て.

I think you don't need to worry about that cause there are many directions in Japan and some people pronounce in different ways.even if you would mix up "su" and "tsu", they could catch with whole sentence.You know what? Most of Japanese, including me, mix up between "L" and "R" in English...

Please take a look at these videos. sa shi su se so (https://m.youtube.com/watch?list=PLsFMhF1eLVn2p1LLFlr9DeHdNbANXwqVH&v=tskK74vaJ1A) tsu (https://m.youtube.com/watch?list=PLsFMhF1eLVn2p1LLFlr9DeHdNbANXwqVH¶ms=OAFIAVgL&v=o39eP6M2VnI&mode=NORMAL)