Decide what size you want to make your matrix. Mine is 21x14: 21 pixels wide and 14 strips high. I like the 3:2 ratio; it's pleasing to my eye, and I think this is a great size for wall art. It's big enough to show some good detail without being too demanding. My finished piece is just over 30"x20" including the frame.
This setup used 10m of pixels just about perfectly. The total came to 294 pixels, which can be comfortably powered with a USB 2A/5V power supply and gave me a little bit of margin for error with cutting.
Cut your strips to length and lay them out on a piece of foam board or poster board. Put LED 0 (with the connector) at the bottom left. Then, lay out the strips in a zig-zag, or serpentine pattern, with the data flow switching directions with every other strip.
To get a perfect grid, I used a strip of LEDs and laid them perpendicularly across the strips, matching up the pixels. This made it easy to get the spacing as even as possible.
Once you're happy with the layout, use clear packing tape to fix the strips to the foam core. Leave about an inch of LED strip at each end un-taped.
Cut the silicone sleeve of each of the strips at both ends in order to expose the copper pads. Give yourself plenty of room to get your soldering iron in there.
Use a hot soldering iron and some solder to tin all the pads on all the strips at both ends. Be sure there's a nice blob of solder on there, but not so much that you're in danger of bridging the pads together.
Grab your silicone stranded ribbon cable wire. Strip off one of the 4 wires so the cable is now made of just 3 wires. Cut a 3" section of ribbon cable and strip around 1/8" of shielding off each wire at both ends. You want just enough wire to fit on the solder pads you tinned. Getting the exposed wire to match neatly with the pads will save you a lot of heartache while soldering the strips together.
Use your soldering iron to tin all 6 wire ends neatly, trim them to be tidy, then repeat. You'll need one fewer ribbon cable than your number of strips (so I needed 13, with 14 strips).
Plug your microcontroller in and turn the switch to "on". Your first strip should light up. Turn the switch back to "off" before proceeding -- you NEVER want to solder strips while the circuit is live. The chances of bridging the pins while soldering is very high, and that's a great way to fry your microcontroller.
Slip a couple pieces of 1/2" heat shrink onto your first connector wire and solder the OUT end of your first strip to the IN end of your second strip, which should be conveniently located next-door. Be sure to get the wires in the correct order: + goes to +, G goes to G, and OUT goes to IN for the clock and data wires.
Turn the switch back on and test this strip before proceeding. Then turn it off again, head to the other end, and solder the next connection. Continue zig-zagging back and forth and be sure each strip lights up as you go.
Slide the heat shrink into place over each of your strips, covering the exposed pads but not blocking the first light.
The lights may have slid around inside the silicone sleeves as you were soldering, so use a ruler to line them up again. Once you're sure all the connections are good and the alignment is right, seal the ends by squirting a little hot glue inside the heat shrink, then shrinking the heat shrink in place with a heat gun.
Be a bit careful here -- the packing tape and foam core are both subject to melting and/or catching fire if you use too much heat gun on them! Use a low setting and take your time.
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