Once your monolith can communicate with microservices, you can start to separate the business logic. A well-organized monolith application may have done this already. For example, if you are using controllers, you want to end up with the following for our previous example:
The reason you want to end up with a structure that separates the business logic is that you can then replace each individual component piece by piece.
So, adjust your monolith to communicate with your microservices and split it up to be easily replaced by them.