Wording when. If you use the correct word, it does not help your word order - and you also want all the correct words - not just one word. Wording will get the correct words and order.

"Be careful to use appropriate wording when writing up the contract." "Word" is singular ("Be careful to use an appropriate word..."), but you need more than one word, of course. "Wording" fits because it doesn't need an article, and it covers the whole style of writing, ie. how the writer chooses and expresses those words.

Be careful to use appropriate words* when* writing up the contract