What exactly is the problem? Of course, a route has to be properly planned beforehand, so that it makes sense and can be entered without interruption. Real there are indeed flight service consultants who learn this profession.
You enter your departure and destination airport on the RTE Page, then the planned route starting with Rwy + SID + route (air routes or direct) and the approach with runway, STAR and possibly a transition. Then check the route on the LEGS page for interruptions, eliminate them and activate the whole thing. Then, once again, adjust the route point by point with the flight plan. This happens then, as mentioned above in PLAN mode and the step button on the LEGS page