At my facility all most nurses chart is vital signs and "no adverse reactions to ABX". I think that's kind of trite.
I say tailor it to whatever they're on ABX for. If it's pneumonia, chart on lung sounds, that the head of the bed is up, if they have a productive cough. If it's a UTI, chart that you're pushing fluids, the color of the urine, if they have urgency. If it's for a wound, chart on the drainage, that you changed a dressing, etc.
The fact that you administered the ABX is already in the MAR.
Depends why they're on antibiotics and what antibiotics/what route, etc. Someone who takes chronic antibiotics for something like acne doesn't need much special documented there. Someone with a massive infection on triples including things like vanco and/or gent that need levels monitored need labs monitored/documented, etc. Someone with a skin infection/abscess should have wound measurements/descriptions documented. The OP is too broad to properly answer it.