You can't 'change' the nature of the stitch itself. If you knit a row then purl a row, you get all the knits on one side and the purls on the other side - is that what you want? If you make a scarf like that, the edges will curl, so you would keep 4 or 5 edge sts in garter stitch, which is knit every row. So after knitting about 5 or 6 rows, on the next row you knit a few sts, then purl to the last few sts. Knit the next row, then do the knit a few, purl to last few and knit. Keep repeating until you've got enough yarn to knit the last few rows.