You should capitalise the first letter of each word, so that people using screen reader software can hear each word spoken separately.

#ThisIsHowToDoIt would read as "this is how to do it"

#thisishowtodoit would read as gibberish because the screen reader software can't tell where the words begin.

indeed. Does a screen reader have trouble with a #one #word #hash #tag if all are lowercase?

Ahhh, sorry! 😳

If it's one word, it doesn't matter what case it is in because the screen reader will read it out as one word.

Only thing to do is not put random capital letters in the middle of the word, because that would make the reader interpret it as multiple words.