I started by tracing my brother's tie onto an old T-shirt.

My shirt was no where near as long as the tie so I had to cut it out into three different peices.
Since I'm making a Gryffindor tie I did two yellow and a red peice.

Next, I hemed the big yellow peice... the front, fat end of the tie.

Take whatever color you used to cut out the middle peice, for me that means red. Sew the two peices together, and since you hemed the yellow, the red will be slightly larger. Make sure you center the red so that both colors will be even when you hem the red. Do the same for the last peice.

Now, get a few red and yellow scraps to make the stripes. Sew the red strips across the yellow tie peices, and the yellow strips across the red. You can decide how thick you want your strips and how far apart you want them to be. Since you add the different colored strips it's very difficult to tell where you switched colors in the original tie.