When should I use "pissworm"?

Can I say "start about" instead of "start with" here?

Unfortunately, "start about" can not be used here. Instead of "start with", you can say "discuss" here.

When should I use "pissworm"?

"Pissworm" is an insulting term that should not be used.

How can I understand "up" here? What's different between "call me" and "call me up"?

To "call someone up" means to call them by phone. It has the same meaning as "call me" but is used much more casually and informally. Ex: Why don't you call me on Friday? Ex: Why don't you call me up on Friday? Ex: I'll call you up when I know the answer. Ex: I'll call you when I know the answer.

Does "to go back over" mean "to reconsider"? Can you tell me how to use this phrase?

Yes, that's correct. In this context, the informal expression "to go back over [something]" means to reconsider, renegotiate, or discuss it again. Ex: I don't want to go over this again. This discussion is closed. Ex: We went over the contract again and came to a new agreement.

What does "look down on someone" mean?

"Look down on someone" means to believe or think that you are better, more important, or superior to someone else or other people. You regard someone with scorn or disdain when you "look down on someone." Ex: I feel as though he looks down on me every time I speak. Ex: She looks down on me when I make a mistake. Ex: Celebrities look down on regular people like me.