Cast on a little more stitches than the number indicated in the knitting gauge you should hit on the indicated needle size. E.g. if you are going to have 23-24 m/10 cm, I would cast on approx. 32 m. Knit 3-4 rounds of garter stitch back and forth, then continue to knit in stockinette stitch except for the 2 outermost sts (the beginning and end of the round); knit these sts every round. Knit like this until you can measure the knitting strength, possibly until you have a little more than 10 cm if you are going to measure in both directions. Choose whether you want to drop the sample patch or line it up. If the knitting test does not match the number of stitches per 10 cm, and you have more stitches than you should have, you can go up a needle size. If you have fewer stitches in the knitting test than you should have, try a smaller needle size.

Good luck!