When you say it looks messy, do the results resemble each other? Like maybe it's the same issue each time? Or have you been able to identify which step you're on, when things go bad? Sometimes it helps to just go very slowly, and watch the status bar with each step and each click.
Do you use a custom document template, by any chance? If so, make a new layer, and try the tiling on the new layer. Or if it's not a custom template, start with a brand new fresh document anyway. There's one particular bug, that can really make tiled clones go wacky, that's related to various kinds of transforms. So if you have a document where you've been playing around, clicking away, undoing, clicking, moving, rotating, etc. any kind of transform. It causes the tiling to go astray. So either a fresh new document, or if you use a custom template, just a new layer, will avoid that bug.