Start with solid dark green color.

Create new layer, switch to Pen tool, make a leaf shape (it will look better a bit unsymmetric, so do not try to make it perfect symmetric).

Apply the following layer effects to the layer with leaf shape:
- outer glow - black, overlay, 50-70%, 10-15px
- inner glow - white, overlay, 20-40%, 15-25px

change foreground color to darker green (almost black), create new layer, and then, using same Pen tool, start drawing...

...drawing...

...drawing...

...until the leaf is ready!

Apply the following layer effects to this layer:
- blend mode = screen
- outer glow - black, overlay, 10-20%, 5-8px