If you are in search of manually merging your PST files, I can suggest you an articulately framed blog. It offers a step-by-step guide to merge PST files.
https://www.shoviv.com/blog/how-to-merge-pst-files-manually/
However, if you are not looking to manually merge your PST files, I can propose a great tool to fit your requirements.
A tool called the Shoviv PST Merger Software tool.