It's all about preparation.

Like this? I am preparing to start my daughter's Dale of Norway "Park City" sweater. I have been swatching and testing yarn. Above is what I made in Hauk and which I think I will end up using.

This is easy to do. Simply find a charted pattern of approximately 25 stitches for each side and work it in the round for however long you need. I think this chart work was 27 rows and then I added the stripes. Work the four decrease until it's the size you want for your top opening and voila! This particular chart came from Vogue's Stitchionary 3.