In my opinion Fpl to IF is a lot easier to use compared to Sim brief. its a lot more structure. Another website you could use is flight plan database. I use that for all my long haul flights and it works really well!
What I do is I download it to SkyVector and then export it as a .FPL file which I open in IF when creating my flight plan.
It works 99% of the time (there’s a very very rare chance that a couple waypoints will be missing but it usually just takes me an additional 2 mins to fill them in since SimBrief shows all the fixes on a map anyway).
You just have to remove airways. Airways are usually a letter or two followed by numbers and will usually be every other word.
Basically, how these routes work is by saying: [waypoint[ via [airway] to [waypoint] via [airway[ to [waypoint].
i.e. the first part of your route is: SAREX W6 NLG W18 TAMOT
That means: from SAREX on airway W6 to NLG, then on airway W18 to TAMOT
You might see DCT which just means “direct” instead of an airway. Delete that too.
You need to remove the SID and STAR from the start and end. A word followed by a letter and a number, e.g. SAREX1L is a SID/STAR. Usually I’d get rid of the first and last waypoint if they are clearly the waypoint the procedure was built off. e.g. SAREX is clearly going to be at the end of SAREX1L.
Use DAL OFP and scroll down to the bottom, there is a fix list. Just copy paste that. You don’t have to use all these additional resources people suggest.