Knitting letters

I hope someone can help me…I am new at knitting and i have a scarf almost done. I wanted to knit in 3 greek letters in to one end of the scarf in a contrast color and a couple of stripes on the other end. My questions are: 1. How do I knitt the letters, i have it graphed out and 2. How do I go back to the other end and put in stripes? Thank you for and help, remember i am new at this:)

If your scarf is done in stockinette st, you have a knit st background. You could do the letters in purl sts against the knit background in either the same color (subtle) or a different color (more contrast). The letters in a different color can be done using the fair isle technique or intarsia, depending on how large you want the letters to be. (There are videos for both under the Free Videos tab at the top of the page, Advanced Techniques.)
Another possibility is duplicate stitch, which is embriodered onto the finished background. You could use duplicate st for the stripes at the other end as well.

Be aware, though, that the letters will be reversed on the back of the scarf.

If you work intarsia, it will look a bit messy on the back, as well. Stranded will be extremely noticeable.

One way to avoid this on a scarf in progress to possibly knit the scarf as long as you want it to be, and then work a folding row–purl if it’s on a knit background. Then knit this section long enough to fold up and sew to cover the back of the letters. You could even make it a pocket! You can do the same with the other end–pick up stitches along the edge, add your stripes and make a similar pocket.

When I’ve made scarves with colorwork, I’ve made them double wide and closed up the long end, or worked them in the round, so all the messiness is on the inside.