Question: how do I buy alcohol?

Hi all! I’m hoping someone can help me with this. I know this question is gonna sound strange but I’m 22 and from Brussels but I’ve never bought strong alcohol in a supermarket. I wanna go to the Delhaize to buy some liquor but most of it is either behind glass and/or has a red security cap on it. How do I put one in my cart? Do I ask someone who works there to open the cabinet? Do I have to show my ID? What about the red cap thing? I can’t do self check out with it I suppose…?

Can someone guide me through the procedure? I’m asking cause I have some social anxiety and don’t wanna look like an idiot in the supermarket by doing the wrong thing. :/