Deform is only available on objects that have meshes. Deform can deform meshes.
Deform edit is only avaiable when you are editing parameter.

Deform one-by-one vertice.

• Select: Left Mouse

Deform path (temporary).

• Create/Destroy:Left Mouse(x2)
• Switch Mode: TAB, Toggle locked point: Ctrl
• Move point along with the path: Shift

• Define paths
• Transform path
• Set rotation center
• Move points along the path

• Deform: Left Mouse
• Snap to X/Y axis: SHIFT
• Select vertices: ALT

• Size
• Brush format
  • Circle Brush
  • Double Threshold Circle
  • Rectangle Brush

• Drag mode
• Flow mode

• Try to not make negative mesh area (e.g. one vertice from external left side is more right than right external vertice) if you don't intend to. It can give you headache trying to fix it later.
• If deform you want to make is overcomplicated, you can make guide art layer to help on deform work (it's better to make guide when you are on cutting model work, but you can also do after you started rigging).