I think you'll find everyone has a different opinion on this.
At the most basic, I basically just pulled each of my multiple repos into a single monorepo - and treated each repo as a source root (in Pants terms).
So, other than the fact that the code was all located in one repo, each sub-project didnt know any better. That's a decent start, and as you get more familiar, you might split out into other patterns, sharing code, etc