Information Extraction (IE) systems are traditionally domain-specific, requiring costlyadaptation that involves expert schema design,data annotation, and model training. WhileLarge Language Models have shown promisein zero-shot IE, performance degrades significantly in unseen domains where label definitions differ. This paper introduces GUIDEX,a novel method that automatically definesdomain-specific schemas, infers guidelines,and generates synthetically labeled instances,allowing for better out-of-domain generalization. Fine-tuning Llama 3.1 with GUIDEXsets a new state-of-the-art across seven zeroshot Named Entity Recognition benchmarks.Models trained with GUIDEX gain up to 7 F1points over previous methods without humanlabeled data, and nearly 2 F1 points higherwhen combined with it. Models trained onGUIDEX demonstrate enhanced comprehension of complex, domain-specific annotationschemas. Code, models, and synthetic datasetsare available at neilus03.github.io/guidex.com