計画を具体化することが難しい。

English Translation

It’s hard to put plan into concrete shape.

ことが, especially the particle が, sounds unnatural although it’s understandable.

計画の具体化は難しい。(けいかくのぐたいかは難しい)= (lit.) Crystallization of a plan is hard.

計画は具体化するのが難しい。= (lit.) A plan is the one that is hard in the process of crystallization.

計画は具体化する過程(かてい)が難しい。= (lit.) The hard(est) part of a plan is its execution process.

sound much better in Japanese.
Particle は is often used when implicitly compares two different situations and highlights the difference – in this case, “easier said (planned) than done” is the connotation behind は.

具現化(ぐげんか)also works and is interchangeable with 具体化.

  • 具体化する = 1) to make something more detailed (e.g. breakdown a high-level blueprint into a step-wise action plan); or 2) to make something happens.

  • 具現化する = to make something happens; to make a mission/idea be realized as a form of tangible deliverable

具 means “content”, “situation”, or “ingredient” (of a soup, for example). 体 is “body” and 現れる is “to appear”.