How do you use a push brush for eyeliner?

  1. I bought this brush a while ago:

    I use Bobbi Brown Long Wear Gel Eyeliner, and have been using the BB brush the SA advised me to buy. I still struggle and have to touch up my mistakes every single day. I've heard these push brushes make eyeliner much easier but I don't know how to use it! Can anyone help?
  2. what is the BB brush? I use that brush but from trish mcevoy and its good. I kind of wish i bought the angled one though but this was the one the sa suggested.