If you want to see a lawyer, you make an appointment with them.

[Rajam]

For example, if you want to see a lawyer, you make an appointment with them.

Is it okay to say this? Or do I have to say, if I want to see lawyers, I have to make an appointment with them?

[Beeesneees]

Because it is unclear from the sentence whether the lawyer you are seeing would be male or female, then in modern English it is okay to use 'them' as a singular personal pronoun. This avoids the awkwardness of having to write '... with him/her'.
Be aware that some exam authorities in countries where English is not the main language may not accept that idea, though! Though most are progressive, a few of them are very definitely stuck in the past.
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Cheers m' dears!