Well, in general, you want to...
1. Set your shutter speed to the max sync speed, which I believe for you is 1/180 (that's what it is for the X-T1).
2. Set your ISO to 200
3. Set your aperture to something in the middle, say 5.6.
Set focus manually so it stays consistent. Take a test shot without the flash, to see how much ambient light you're letting in. Too much? Hit up your aperture. Not enough? Open the ap up. Then set your flash to some kind of middle power setting, place it, and fire off a new test shot. Adjust the flash setting up or down, move it, whatever. But understand that as you adjust shutter speed, you're only affecting how much of the ambient light makes it into the picture, not flash. And as you adjust the ISO and aperture, you're affecting how much ambient AND flash will be captured.