How do I do 24 sts and 36 rows = 4" in garter st ?
This is the gauge information for your project. You can do a test swatch by knitting a 4 inches square to see if your gauge and the pattern gauge are the same.
You can do the garter st swatch by cast on a little more than 4 inches so you can measure it easier. Try CO 30 sts and then knit every row for 4 inches. When complete, measure number of stitches in 4 inches and number of rows in 4 inches. Compare these # to the gauge given above (24 sts by 36 rows). If you found your gauge to be little bigger or smaller than the given gauge, then try using different needles. For example, If your gauge turned out to be 20 by 34 instead of 24 by 36, you can change to smaller size needle to get more sts and rows.
Usually I don't worry much about the row part because I can always add more rows if I need project to be longer.
Good luck on your project and Thank you for the question.