It's the pattern of knit and purl rows that'll give you the yoke pattern (sort of a plowed field look). You don't need other yarn. Just follow the directions for the first 10 rows, repeat them the number of times you need to for your size and you'll start to see the pattern emerge. Put a safety pin or a loop of yarn on the right side (the side that will face out on the dress) so that it's easy to remember and keep track of which row you're on with a pencil and paper.
For the first part of the yoke you'll knit back and forth. Then you join the edges so that you're kntting in the round. The directions are pretty good and will be clearer when you have the knitting actually in hand. That back and forth part gives you the opening for the buttons. From there on, the yoke is knit in the round. Here's a video that may help when you get to joining to knit in the round.