Well, I am a chinowner for only 2 months, but I can share you my experience.
I have bought a FN aswell for a 2 and 4 month old chin. I went to get some mesh wire to ataround the cage. I measured the wire panels and cutted the mesh wire to the apprpriate size.
I attached everything with plastic zip ties. I knew this was a risk, but tried it anyway. The chins never showed intrest in them, and when I removed the wire last week all the zip ties were still there. This depends your chin I guess and plastic isn't the way to go but it worked for me.
People advice to attach from the inside to prevent a small chin getting stuck between the wires from the FN cage and the mesh wire and thus hurt themselves or worse.
I considered this, but I wanted to avoid fur damage from the mesh wire so i attached to the outside of the cage. I countered this by attaching it every few cm. Although it is maybe not necessary I prefered to play safe and to give you an impression: 600 zip ties were used in the process
Another thing I stumbled upon here on the forums that I nearly forgot myself: Wire the top panel (ceiling) aswell as kits can climb up and squeeze through. I did the same with the ground panel.
When to remove the wire? I removed mine as soon as my youngest one weight over 350 gr. He started to jump around more and i was afraid he could get a foot stuck between the mesh and break it.
The chins needed a day or so to adapt to the mesh free cage