I'm not entirely clear what your goals are but roughly, just figure out an application that holds your interest and build a model for it from scratch. Probably don't start with an LLM though. Same as for anything else really. If you're interest in computer graphics then decide on a small scale project and go build it from scratch. Etc.

Maybe look into model finetuning/distilation. Unsloth [1] has great guides and provides everything you need to get started on Google Colab for free. [1] https://unsloth.ai/