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I have string
String levels are also handy, you can make as long a level line as you can make a taught piece of string.
https://www.screwfix.com/p/stabila-builders-line-level-3-80mm-/5494x
A length of timber with a normal level that you (probably) already have will do the job and will also help with alignment. Set them out and measure corner to corner diagonally across the pattern. When the measurements are identical both ways they're perfectly square. Use a string line (any old string stretched tight between two pegs will do) to set out one side of the pattern and work away from that.