If you know how to do rib stitch you also know how to do seed stitch. It’s basically the same but you start with the other stitch.
Check it out.
Now put into practice all the knitting you’ve learned so far.
Cast on some stitches on your needle. For instance 35 or 40 stitches, stretch them a bit on your needle to see if that’s the width you’d like your scarf to be.
If too wide, undo a few. If too narrow add a few more stitches.
Knit as many rows as you need for a scarf in seed stitch and when done bind off the piece as you learned above.