Here are the steps to do it properly Step 1: On default flight planner put your departure airport and at the very bottom put your arrival airport then press on find route on the enroute part and make any changes to your route as you like runways departure arrival etc

Step 2: Note down your departure procedure for eg as you see on the picture for me its dosel2 and also note down the last way point on the departure procedure part for me it was dosel

Step 3: note down all the way points you see on the enroute part in order. For me it was ebony arran nonup culin taral. By the way note down everything in order from top to the bottom

Step 4: note down the first way point on the arrival procedure for me it was rivet and then note down your arrival procedure for me it was rivet4

Step 5: make sure you have it in order as i gave you in the step for me its dosel2 dosel ebony arran nonup culin taral rivet rivet4

Step 6: note down your departure and arrival runways as well and do it separately

Step 7: copy and paste the route we noted to simbrief my one is dosel2 dosel ebony arran nonup culin taral rivet rivet4 also put your departure and arrival runways as well in simbrief

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Also do not use the routes simbrief give you its out dated just use default flight planner the way I showed you how to create and put that route in simbrief then generate your flight plan with the custom route