You should cut your first squares at 7 & 1/2" square to make your unfinished HSTs at 6 & 1/2" (finished at 6").....but then you likely will need to trim them down to the exact 6 & 1/2" size. Everyone's 1/4" seam allowance is a bit different so this method of starting at 1 inch larger than you need is a good method of being sure you will end up with an accurate HST.
You don't "need " a tool to make HSTs; just don't forget the trimming!
Jan in VA