https://opensourceconnections.com/blog/2023/02/27/hybrid-vig..., https://jina.ai/news/hype-and-hybrids-multimodal-search-mean... are two I've been looking at

Basically, get the BM25 results and normalize them to be between 0 and 1, then take the (potentially weighted) average of them and the cosine similarity results (already between 0 and 1) to get the final ranking.