If you make yourself at home, it means you behave as if you are in your own home. If I make myself at home at IKEA, it might mean that I use the furniture as if it were mine; perhaps I lie down and take a nap on the bed, sit in a recliner and put my feet up and read the newspaper, sit down and eat a snack at the kitchen table. As Su.Ki explained, "very much" emphasizes how much the customers are making themselves at home.

The problem here is that you are dividing up the sentence wrongly : “make oneself very much” means nothing at all. The 'very much' doesn't go with 'customers' - it goes with the phrase 'at home'. The basic phrase here is: "Customers make themselves at home." The adverbial phrase 'very much' is modifying another adverbial phrase 'at home.'. It means that the customers are not just making themselves 'at home' - they are making themselves 'very much at home'.

You need to provide the rest of the sentence. Does it end with an adjective?

Is the whole sentence "Make oneself very much give up the idea forever"? I think "very much" here modifies "give up. so i guess the meaning is: it is more likely you will give up the idea forever.