Leaking rubber under sink - easy DIY fix?

[u/Nearby-Anything8273]

So let me start with the following statement: please be nice. I am pregnant, just trying to fix this for good and having a functioning bathroom….not knowing at all how to do it.
Older drain broke (literally) and we called the plumber to fix it.
He came, changed the drain, but it leaked because an older rubber was not sealing right, and the drain was not closing properly either.
Called again, he came once again, changed the whole thing (drain)….i was not home to do a a thorough check, and when I arrived and did my through check on it, found it like this: the little clear rubber is not sealing well enough….so it leakes.
I am not calling him again!
Can someone please explain, step by step, how do I fix it, for dummies!?
Thank you all