It depends on whether you want to access the Web service directly or via MBOs.
If its the former, its pretty straightforward, but you'll need to do a lot more manual coding, won't have offline support, et. al.
If its the latter, then its pretty much the same as if you were creating MBOs whose backends were SAP or DB. There should be plenty of online documentation to discuss how to create MBOs with a REST/SOAP backend. If you have reviewed it and have specific questions, please feel free to ask for clarification.