Please help!

I am trying to install my vessel sink but I can't stop it from leaking at the base of the sink. What the heck am I doing wrong??

I put plumber's putty under the lip of the drain, inserted the drain into the hole, attached the gasket and nut directly underneath. How do I get it to stop leaking?
Do I need to put plumber's putty on the gasket too? I don't think I over tightened.

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Thanks in advance,

Angela