It takes some time and trying different ways to attach the thigh parts. Your armor and how it's trimmed will also play into the fit. The picture you posted looks like the shins are trimmed too much (riding high) and the thighs are riding too low. It actually looks like a 6" trooper in armor designed for someone around 5"7-ish.
I'm 6"2 and this is how mine looks with AP armor: