Transfer learning has transformed natural language processing by enabling pretrained language models to be adapted efficiently for downstream tasks. This paper provides a practical framework covering feature extraction, full and partial fine-tuning, adapter-based learning, prompt tuning, instruction tuning, and zero-shot inference. It discusses leading transformer architectures including BERT, RoBERTa, DistilBERT, BioBERT, SciBERT, GPT, and T5 while outlining implementation strategies such as gradual unfreezing, discriminative learning rates, domain-adaptive pretraining, and parameter-efficient methods like LoRA and BitFit. The framework helps practitioners evaluate trade-offs among accuracy, computational cost, data requirements, deployment constraints, and governance considerations when selecting transfer learning approaches for real-world NLP systems.