I find the easiest way is to double up on the quotes to handle a quote.
Worksheets("Sheet1").Range("A1").Formula = "IF(Sheet1!A1=0,"""",Sheet1!A1)"
Some people like to use CHR(34)*:
Worksheets("Sheet1").Range("A1").Formula = "IF(Sheet1!A1=0," & CHR(34) & CHR(34) & ",Sheet1!A1)"
*Note: CHAR() is used as an Excel cell formula, e.g. writing "=CHAR(34)" in a cell, but for VBA code you use the CHR() function.
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