I found the 2 hardest parts of the rails to be first getting the shoulder cut straight. Even 1/2mm out and it looks naff. To fix this I made a cutting guide with magnets to get them straight. Hopefully, I'll get better at just cutting them straight from the saw. I don't have a shoulder plane to help either. Secondly getting the dovetail bit flat and level. On practice attempts I found mine to be lumpy. After a few goes, I realised that I needed to make longer, lighter strokes with the chisel.