How can I set a graphene sheet in the center?

Deer everyone

I'm a beginner and I follow steps decribed in "https://community.accelrys.com/message/10647" for making a graphene sheet.

I'm alomost done.. however there is a difference between my picture and that of the master.

1.PNG2.PNG

I would like to set a graphene in the center as like the left picture.

Thanks to your help!!