how remind a professor we were supposed to have a conversation through Skype? Recently, I am looking for a PhD position.
I emailed a professor and he said I chose a day for Skye, I did.
Then he said that afternoon is OK with him, and asked me a bout time difference between my country and his country.

then he did not answer me for 3 days and yesterday was the day we were supposed to have Skype.
now, I do'nt now how to remind him
This is a starting point.  You can adapt this to your own style.

Dear Prof. X,
I hope all is well with you.  Did I misunderstand when our Skype appointment is?  I thought it was yesterday.  Maybe I should make sure I know what your skype ID is.  Is it profx?  Mine is maryam.
If you have time next week -- days that would work for me are m, n, or p.
I'm attaching my unofficial transcript.
Sincerely,
(name)