Changing colors: How do I get it on the needle?

[HamaLee]

I am knitting an iPod sock for my cousin...and it has a cute little pocket on the front. I want to try and knit her initial on the pocket in a contrasting color...and I think I understand the basics of both Intarsia and stranding from Amy's video except for one key point: How do I get that contrasting color onto my needles in the first place?

I've knit about an inch in the primary color, b/c I want the letter to be in the middle...so how should I get that contrast color on there? Should I have cast it on right from the start somehow? Do I cast one one stitch at a time and knit both colors together?

Thanks!