Ummmm, badly? My flirting is really subtle (probably to the point of being unnoticeable). It usually involves witty banter, lots of sarcasm, and just joking around constantly.
Quite frankly, I'm awful at emotional things...I don't know how to express my emotions outwardly without it being awkward. I feel a lot and think a lot on the inside, but getting those feelings to come out is a very rare occurrence, and usually makes me slightly uncomfortable -- and this occurrence has to happen naturally with someone that I am VERY comfortable with and trust a lot.
If you have ever gotten an ISTJ to "spill their emotional guts" to you, consider yourself very blessed. They'll likely stick around as a friend (or more) for life.
As for flirting through being seductive or something, you can probably forget about that. We usually don't stand out in a crowd, and unless the ISTJ you're talking about is smokin' hot and actually has the confidence to go along with it, we probably dress more on the conservative side.
I'm really trying to think here...I know what I don't do, but saying what I do do is very tough. Usually the people that have asked me out in the past just said that I was "different." It's probably that I don't try and stick out...I'm just myself and don't try to force things. I never take initiative. I figure if something will happen, it'll happen eventually. Kind of like the whole "waiting" thing that was mentioned in the Male ISTJ thread.