Answer: The most accurate way is to set out posts with string running taut between them to mark the desired heights and widths. Or, the gardener may be following a benchmark on the house such as window sill height, if it is along a foundation. When you prune, prune slowly and step back often to check your work. (You can always prune more off, but once cut you can't put it back on!) Pruning in topiary form takes some skill which gardeners learn over time. Also, really intricate forms often have a metal framework inside to help the gardener know exactly where to cut. I hope this helps.
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