Tactics vision OTB

TL;DR: I am unable to spot tactics over the board.

I learned the rules of chess online. I practiced tactics using chesstempo. I have 1900ish tactics rating on chesstempo.

I purchased an actual chessboard quite recently and today I played with my cousin and I placed two pieces such that they got pawn-forked!

I felt quite disappointed with myself.

So how do I translate my two-dimensional tactical vision to a 3d board?

Practice games or practice tactics or both?

Programmed instruction, and set up a real board for each problem. It takes a bit longer than a digital board but it could still be argued as a better use of your time since you will likely reap greater improvement on a per puzzle basis. Anecdotally, I didn't really make significant progress until I did just that. Winning Chess by Chernev and Reinfeld was the book I used and they encourage that exact method.

No kidding. Personal computers didn't exist when the book was written.

You know, I hadn't considered that! And yet, as a testament to the efficacy of the method, people still managed to reach master level even back then. 

True, but one could also note that people are making master at younger ages these days. Just because Reinfeld/Chernev recommended using a real board doesn't necessarily mean that real boards are better than digital boards. It may or may not be true, but you can't use Reinfeld/Chernev as an argument to settle the issue. You have to look at other data/experiences.