There is no official way to fly fix to fix IFR without some form of RNAV/GPS, unless both fixes happened to be on an airway which can be flown using an NDB QDM/QDR, VOR radial or something.
Obviously you can do some form of DR until you happen to intercept a VOR radial or DME arch which also cuts through the fix, and then follow that. But that will not be what ATC is expecting.
And that brings me to the most important point. Obviously you are going to plan/file your route so that you can fly it with the equipment you have on board. No GPS/RNAV capability? Then don't plan to do fix-to-fix ("DCT"). But as soon as you start the flight, especially in busy airspace, the plan goes out the window and you should expect deviations from ATC. Vectors are fine, but if you get a "direct X" without the ability to navigate that with reasonable accuracy, you should simply refuse. In theory, ATC should be able to glean from your equipment list that you don't have RNAV capability so they should not give you those instructions, but in practice, they assume everybody has RNAV capability of some sort and can do a "direct X".
SDF? Had to look it up. It looks to be some sort of navaid that's only used for approaches. Not for en-route navigation. And there's only one location in the world that still uses it.
https://en.wikipedia.org/wiki/Simpli...ional_facility