The pdf is here that you can download for free
Tension or gauge is the number of stitches and rows that results from a hook and yarn.
~The tension above is for a 4mm/G hook and double knitting/light worsted yarn.
Not everyone crochets at the same tension so if you want to make sure your garment
works out to the correct size you would need to match your tension to the same as the designer.
Make a 5cm (2”) square in the stitch provided in the tension/ gauge so for this it would be tr/dc (depending if you are using UK/USA terminology)
~I made a larger square to show you, this is 4 rows so that is correct tension for the pattern.
If the rows were less, say 3.5 rows you would draw less yarn after inserting the hook to make your rows, to make them shorter, or draw more to make them longer if you were getting say 4.5 rows, you may need to practice this.
~This is 9 stitches so that is correct tension for the pattern.
If the sts were less, say 8 you would try a 3.75mm or 3.5mm if you had more stitches (10) you would try a 4.25mm or a 4.5mm hook and make another square to check the tension until you get it correct.