I've edited all the frames of the wraith, but how do I compile them into a .GRP? When I click the compile to GRP button, it only lets me select the .ral, which gives me no frames.
That's right. The ral is a list of filenames, which should be the names of your frames. Just make sure that they're all in the same folder, select the ral, and say compile.
But when I select that, all I get are two black frames labeled 0 and -1.
That's because it doesn't show it to you immediately. RetroGRP is kind of stupid that way. You'll need to open it again to see the edited frames.
Oh, it adds the _x to the end.