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.
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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.
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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”.