Start by doing a normal Dip on Parallel bars or a Dip station .
When reaching the bottom position, lean back until your forearms rest on the bars.
Then push back into the bottom dip position, and push yourself back up into the starting position.
Form tips
Good Shoulder extension backward flexibility is necessary to reach a deep dip position and be able to transition to the forearms support.
Let your legs extend forward as you lean back on your forearms; bringing them in when you flip back into the bottom dip position will help with the transition.