I agree with mortal, the simplest way these days is to use GPxTrack (one of the tabs in GPxPatch).
Take the track you have downloaded, and place both the
.dat and
.wad files together into a new folder somewhere. If the downloaded track also contained a .gpi file (menu trackmap) and/or a .bmp file (loading screen), put those in the folder as well (but these are optional).
Now go to the GPxTrack tab, click Edit, and then in the box for Circuit 12 (or whatever) navigate to the .dat file in the folder you created.
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Intel NUC 8i3, 8GB RAM, MS Sidewinder Wheel