how to write tsu & shi in katakana

the letters "shi" and "tsu" look very similar in Katakana. How do see and or write the difference ? (also for "n" and "so")

Well. If they submit how faces... The first face "shi" is smile. And the second face "tsu"is laugh. Also in a writing. With "tsu" near stroke longer than that of "shi" and bays up. Difference same us the "smile" and "laugh". Can not be seen only from far. Trust me, after one year you can differentiate this simbols, as white and black. So, dot break you head...

a Japanese guy taught me this, and it made sense so I can always remember:
you know how shi is written in hiragana, right? down & then up, kind of like a 'U' shape? Replace the 'down' stroke with two dots, and then up...that's your katakana shi. The two dots would cross the down stroke, not be parallel to it.
For the 'tsu', it's the same...the hiragana 'tsu' goes from left to right, and then curves under & around like a sideways 'U', right? The two dots cross the first half - the left-to-right line, and then the curved line takes over & goes down from there.
This all makes more sense when you try writing both, and it makes more sense if you are already familiar with hiragana. I learned hiragana first so this worked for me.
'n' and 'so' are similar but it doesn't fit quite as well...if you know hiragana, try replacing a bit of the hiragana with the dot, and follow through with part of what's left...see, not quite as easy.
Anyway. Worked for me.

thank you all.