You should consider what makes someone sound condescending.
I know – that's pretty much what you're already asking. But I really think it's what's at the heart of your question.
One aspect of people talking to them that a lot of people (especially adults, but perhaps surprisingly, also not uncommonly children) tend to find condescending is when someone either (a) assumes that the person they are talking to doesn't know the subject matter, so they simplify to the point of almost ridicule or even tell them some variation of "you wouldn't understand"; or (b) want to show off their knowledge, so they make a deliberate point of being overbearing, typically by using terminology or phrasings that the person they are talking to cannot be expected to know.
In contrast, someone who genuinely wants to share their knowledge in a civil manner is likely to try to quickly and fluently adapt their communication to the person they are talking to. If the person they are talking to seems to understand things or even just expresses interest in the discussion, this means to go a little more into appropriate depth; if they seem puzzled, to explain a little more and maybe use fewer field-specific terms. If the details are important, they'll probably focus on explaining over simplifying; if getting the general point across is more important than the specific details, they'll probably focus on simplifying.
Therefore, if you want a character to sound condescending, I suggest to either have them over-explain everything in very simple language, or to knowingly speak over the head of the character they are talking to. In both cases, they should also ignore hints that the character being talked to doesn't understand, or even explicit pleas for clarification or more details. They may or may not more or less outright refuse to adapt their style of communication to match the character they are talking to; some variation of "I shouldn't have to explain this to you like you were five years old" once in a dialogue can go a really long way, but it shouldn't be forced.
If the POV you're working with allows it, you can also add some inner monologue for the character that they are talking to, indicating that they either don't fully grasp what's being said, or that they wish the character talking to them would explain in a bit more depth.