Maggiicc, but for real I dunno,. Are you talking about how he gets there originally to become Lord Braskas Guardian?
I'm not exactly sure. We're told that Jecht left "Dream Zanarkand" and never came back. In that time he was gone, he was Lord Braska's guardian along with Auron. Well, it could be that "Dream Zanarkand" was sort of projected over the real, deserted Zanarkand. That'd make more sense. I guess that's one of the more blatant plotholes in FFX.
I'm pretty sure Dream Zanarkand was a projection over the real world, yeah. If I remember what I've read correctly it was somewhere far out to sea, far enough that it was never really found by the common folk. I assume Sin's attacks kept people from properly exploring. Hence Jecht got lost when he was out sailing one day, and washed up in... Besaid, I'm pretty sure? So I assume Dream Zanarkand was somewhere near Besaid, since that's where Tidus winds up as well.