Step 1: Stop seeking advice about women on Reddit, where 99% of the populace are either basement dwellers who have never met a single woman in their life (aside from their moms), or jealous, sexually frustrated landwhale women who are no example for anyone, because the only advice you can ever get from them would be "make them ugly as shit just like me, or else I can't self-insert as them and you're a terrorist".
Step 2: Try to make them either as cool or as funny (or both) as you can. No one will complain about a character's sexiness if they're entertained by more than just that.
Step 3, AND MOST IMPORTANT ONE: Might sound counterintuitive, but it should never be about her. IT'S ABOUT THE POV CHARACTER LOOKING AT HER. Think about it for a bit. Why is she sexy? It's not like God or the universe decided she was universally sexy, was it? What measure is there for 'sexy'? There is none! There are many women who peers of mine said were complete bombshells that to me just looked totally average, and vice-versa. Everyone has their one type.
Thusly, have the POV character describe her as sexy because THEY think she's sexy and are attracted to her, but maybe another character might not think so and say the exact opposite right after that.
IF SHE IS the POV character herself, then it's even easier, because inevitably some of the chars she meets will think x of her, and others will think y, just like in real life.
Human interaction 101, baby.
Hope it helped a bit. :)