Get the slide/frame to fit most of the way together, then lap to fit using 600 grit (or so) lapping compound. A knob shaped doohickey from Marvel fits into the bushing cut and acts as a handle to push the slide around while the frame is in a vice. For slide/frame that's already mated but needs some smoothing (like an existing factory gun that stinks), use something finer, like JB Bore Cleaning Compound. US Products, now available from Brownell's, makes a garnet based lapping cpd that's non-embedding. I use the US Products cpds, and they were recommended to me by an AMU armorer.