On sheet 2 you have a couple options.
1: Whatever cell you want to show the result in do this: =Sheet1!A1 *where Sheet1! = the actual name of the sheet, surrounded by single quotes. ie('Sales Sheet'!A1) and A1 = whatever cell the COUNTIF formula is in.
OR
2: Put the formula in sheet 2 in the cell you want the result in like this: =COUNTIFS('Sheet1'!M77:M121,"*S*", 'Sheet1'!O77:O121, "20", 'Sheet1'!N77:N121, "flt" *where Sheet1 = real sheet name of Sheet 1.
If the name of Sheet1 is indeed Sheet1, then no single quotes are necessary.