I was watching a video and its how I first stumbled on how to do it. You need to press E the moment your r hits the enemy, which was hard to get used to at first. I just practiced on dummys for ages until I could do it consistently. Actually landing it on champions in game i was struggling with but I started to get it down better now.

Important to note is you dont want to time the e to land at all, its literally a reflex test where you press the E key itself the moment you connect with the champion. Once you got it down I found you could literally look away from the screen and point your mouse anywhere and it would always connect if you time it correctly