Well, simply and first of all, get your vest ready to wear. Once you have it available, put it on and with the velcro (or mark the panels with chalk), attach the panels to the vest, starting from the top and looking yourself in the mirror everytime you do this.
You could make marks with some colored (i recommend BLUE) chalk, the ones they use in school. DO NOT USE A PEN OR PENCIL FOR IT, it could be difficult to take the marks off afterwards. Make the marks where the armor panels should go. This process is to ease the location of the velcro where you want it in the vest. Sew the velcro to the vest and then repeat the same process, but this time the armor will stay there, unless you want to wash the vest.
For the left chest panel, if you're using the light display, you should do an inside pocket inside the vest so the circuit could rest there, and be considerate when applying the velcro in this specific panel, you could find it difficult to install the velcro...just try it.
Hope this helps you, and that you found the answer you was looking for.
Instead of a mirror, I use two pillows, and stuff them inside the vest. That way, I can 'eyeball' the armor placement easier.
Another trick: Fold the front of the vest in half (lining up the sides), and iron a line from the neck to the bottom. This will help you position the center piece and the tummy plate, since both plates have 'points' to follow. The ironed line will eventually wear itself out of the vest.