Funny you ask. We have a contributing guide, but its only ok-ish, working on an update right now
But basically, fork the repo, work on your fork, then propose the PR
There's a CLI tool to help boostrap a package, or you can just copy the closest existing package
For example, if I was adding a new LLM, I would copy llama-index-integrations/llms/llama-index-llms-openai-like into a new folder like llama-index-integrations/llms/llama-index-llms-my-llm and work from there