When to release a pass for the overlapping fullback

In training, we were doing a crossing build-up drill and I was dribbling in and passing to the overlapping fullback and I was releasing it too late and the fullback was offside by the time I passed the ball. How do I fix this?

My coach then gave me a tip. He said I should release the ball as soon as I see the full back crossing me. I was still looking at him when he was already passed me. That's because I was looking at the goalline side rather than towards my own goal side. I need to watch when he goes into my blind spot. That's when he's behind me and probably about to cross the ball and the defense line. THAT'S when I'm supposed to release the ball. 

I need to work on making this small thing habit and I'll avoid putting my teammates in an offside position. 

That's it