Instructions for fine-tuning pre-trained models

Is there any chance that we can have a full description on how to fine-tune pre-trained models (for example in machine translation)? I've managed to continue training on a much smaller dataset (by using the pre-trained dictionary) but the results are disappointing. The model gets kind of "broken" (it mostly outputs "unk"). Am I missing any step? Should we update the bpecodes as well? Thank you!