These are a minimal set of step to setup fitnesse, decaf and run a simple test.

Create a folder eg:c:\temp\fitnesse
Download fitnesse.jar from fitnesse.org into c:\temp\fitnesse
Run java -jar fitnesse.jar -o -e 0 -p 8080 from c:\temp\fitnesse (more options ...)

Install Decaf npm install -g decaf
Point your browser to https://localhost:8080/root?edit and type

!define TEST_SYSTEM {slim}
!define COMMAND_PATTERN {decaf.cmd %p}
!path fixtures

Create a folder for the fixtures c:\temp\fitnesse\fixtures
Add a greeter.coffee with content

class @Greeter
  hello: (who) -> "Hello #{who}!"

Point your browser to https://localhost:8080/HelloDecafTest and type

|import|
|greeter|
|script|Greeter|
|check|hello|Decaf|Hello Decaf!|

Save the test and run it, Hello Decaf! should go green.