I'll have a video up soon as I can to help with this.
Basically you have your inventory, generally you need same sized items or put a smaller into a bigger spot. You would have one rotated horizontally, and one vertical.
Like so: https://cdn.discordapp.com/attachments/597527372727844896/598370256670425088/unknown.png Then you would drag one onto the other and they'd swap places EXACTLY. In current position. So the horizontal one would start in the topleft square and end up mostly off screen. The vertical one would also end up off screen either up or down.
It'll also overlay over other items. My friend hasn't been able to do it, and I can only think it's because he's on Windows 8. And I am on windows 10. Outside of that I can't think of anything else that prevents him from doing so.