What is the best way to photograph lightning storms using an EOS400D SLR?
Using any camera, what you want is a long exposure — minutes, if necessary — to get a sufficient number of strikes that some of them are visible in the frame. It’s essentially the same as shooting fireworks; some digital cameras have a mode specifically to help with that and it would be worth trying for this too. I presume you’ve looked at the two Instructables on this topic…