Stitch small segments of conductive thread between each pixel, connecting the output of one pixel to the input on the next.
Check out our Conductive Thread guide for more tips on working with conductive thread!
Once you've stitched a few pixels, test for shorts with your multimeter (make sure your long power and ground threads aren't touching), and fire up the NeoPixel library test code to ensure your fledgeling circuit is all good so far.