I just hold on the pot and bang on it from the side. Tighten it down as you go so it doesn't move around on you. You'll need to get some big Channellock pliers to tighten it down. The aren't that expensive at the big box store. It made it super easy.
I had this same problem when I built mine. I used the same ferrules from StillDragon. Just screw the ferrule in without the washer and bang on it with a rubber mallet. It won't damage the ferrule, but you can get it flat enough to screw it down with the washers in there. I've done 2 brews since...