There are multiple tasks for doing that
• First of all you need to be almost an expert in REST ApI and Oauth 1.0. Token based authentication used in NetSuite REST is a standard approach. I never made any client, but that I am saying to anyone who is interested in developing external app. My focus is on server (NetSuite) side
• When I work on testing custom Restlet or REST NetSuite WebService, I use POSTMAN for client simulation... That is the very first step. Be sure that everything is fine on the NetSuite side, then start developing external client