how to associate a project to a field scheme

[Nic Brough -Adaptavist-]

It's the other way around.  You associate a field scheme with a project.  (Minor quibble with the question, and probably more about language than the technical points)

Log in as an admin, go to the project, look at the config and find the current field scheme.  Click on the configure link and select the scheme you want to change it to in the list it offers.