My name is Cas and I make cookies to sell. I call my little enterprise Cas's Cookies. Is this the proper use of an apostrophe?
If I put the apostrophe after the second s then it changes my name to Cass (which I am not)
You are correct!
If your name were Cass, then your enterprise would be Cass's Cookies. If you see
s' at the end of a word, then generally the
s is from the plural. For example two cousins who make and sell cookies could call their enterprise "Cousins' Cookies".