How to Parse Arithmetic Expressions — a Data Structures Perspective. This resource seems to be the most popular so far, but there is still a disconnect in his idiom for many students. Let's try to fix that here. When he says to push "? token ?" or "- ?" onto the expressions stack, he actually means to create a parse tree node with the token as its operator and NULL for children. The hardest part then becomes storing as part of a parse tree node not only an operator token, but also a number token. Just add the number as an extra member. It'll turn out useful later during evaluation. I hope that helps!
Parsifal Software's Introduction to Syntax-Directed Parsing