Improving existing project with proper domain modeling

Dear F# Community,
Years ago I wrote a PowerShell module to create Json Web Tokens: jwtPS. The whole project was written in its first version completely in PowerShell. I re-wrote the project roughly two years later in F#. I, unfortunately, have little experience with software architecture and wrote it without any preliminary planning. I am at a point where everything works but I would like to create a domain driven design. I have seen some beautiful examples on Youtube videos or other blogs (Why you should model your domain with F# - Viktor Andersson) but I struggle to translate it to my project (jwtPS).

Does somebody have any tips what I should change in the project to improve the domain design? in this module i see not only types but also many functions. maybe it need move to utils.fs?