I like using the ribbing stitch - which is knit one, perl one - across the row on all rows. You can do whatever stitch you like (garter, etc.), but I think this one looks best with the yarn changes, which I'll explain later.
Simply put, you make every block of yarn a random number of rows. You can see below I cast on 15 stitches of blue, then tied on white yarn after I decided to. Again, this scarf is a random pattern that looks really nice displaying whatever two or three colors you choose.
See fiance in background.
I like to knit a few small rows, a few big. Again, it's all random but be sure to switch off the colors evenly - especially when you're using three colors.
You can see me tying on the blue yarn to a finished white yarn row. Here, you can start to see what I meant earlier about the ribbing looking better for this pattern. If you did a garter stitch, the transition would show in the pattern more prominently. This white piece you'll cut off the ball of yarn, leaving it hang. Later you will tie and sew those ends in to hide them.
Here, I'm simply knitting the white yarn to the blue. You knit it like you would any other row.
Here's what the finished pattern should look like. Notice I've made each row's width completely random. Some are only one or two rows long, others are big blocks. This is not a finished product, though. I still have to sew the ends in.
In the next post I'll show you how to sew the ends in, and where I'll be wearing my scarf.
Do you want to know how to knit something specific? Let me know. Happy knitting!