My hoodie has a long tuxedo-style tail, and I wanted to add a matrix of lights with varied spacing for a twinkly, star-studded look. These lights get sat upon, so I made sure the wires had plenty of flex and play. I also want them to fail gracefully -- one wrong stretch or dance move shouldn't kill the whole tail. I made two independent strands so that if one wire breaks, the worst thing that can happen is that half the tail goes out.
Lay out your hoodie and play around with pixel placement. Decide how many pixels you want and where they should go. Mark the placement on the inside of your garment with a fabric marker (test first to be sure it doesn't show through).
Then, figure out your power/data path. Do you want a serpentine (s-shaped) layout? Or a spiral? Or stripes?
The way you lay out the pixels will affect how your code patterns look, so put some thought in here. For my hoodie, I used two separate strands in a serpentine layout.
Now that you have your distances figured out, it's time to wire up the pixels. Here's a video detailing a good way to do this.
If you're using multiple strands of LEDs, splice the power, ground, and data wires all together at the beginning of the strands. Splice in a very long wire -- long enough to reach your microcontroller with lots of slack -- and cover the connections securely with heat shrink.
Hook the wires up to your LED tester and make sure everything is working and that you like the placement and layout. Once you're happy, secure the LEDs to the inside of the fabric with fabric glue.