Step 1: Import your reference image

Step 2: Reduce reference image to 50% opacity. Create a black background layer. Create some clean line art (White).

Step 3: Hide your reference image to check your line art. You can keep refining it to capture the necessary information.

Step 4: Using the Lasso tool (L), begin creating large blocks of colours for each section on different layers underneath your line art.

Step 5: Create a clipping mask on top of each sectioned layer. Using the gradient tool (on lineal or radial) begin applying some harmonious colours. You can eye drop (Hold Alt in Brush mode) from your reference illustration.

Step 5: Create a clipping mask above your white line art by holding ALT while hovering between the base layer and the new layer. Now you can use a hard edged brush to recolour your line art where you want more contrast from the background colour.

Step 6: Using a soft round brush, create some light flares with Yellow  and Blue  on a separate layer. Change the blending mode on this layer to Vivid light.  (You can experiment with more blending modes to achieve different effects).

Step 7: Experiment with adding some textures to give your illustration more depth and elements. Try applying some Galaxy photos on Screen and see how these colours affect your background colours.

Step 8: Adding noise to your illustration will create a consistent texture suiting for this style. Create a black layer above your layers. Go to Filter> Noise> Add Noise to create a black and white noise map. Click OK. Shuffle through blending modes until you find one that works with the colours (Screen or Subtract work well). 

Set designing: Adding foliage. Using the lasso tool, create some loose shapes of leaves on seperate layers. Give them a solid colour with the Paint bucket. Change the opacity and more them throughout the layers to achieve interesting overlapping elements.