Acer palmatum 'Yves' Arakawa'

Acer palmatum 'Yves' Arakawa'
Japanese Maple 'Yves' Arakawa'

CUTTING GROWN, of course!

Please see attached image from Meriggioli's book Bonsai Maples.  The core principle being illustrated -- thicken a section, cutback to a new leader, thicken next section -- can be used to grow any style of bonsai: upright, slant, cascade, etc.