1) Remover drain stopper/shower drain cover
2) Use a wire hanger to fashion yourself a "drain snake" - unravel top so wire hanger is straight and close top hanging curve so you've got a bit that the gunk can be caught with. Use "snake" to fish into drain and pull out as much obvious gunk as you can.
3) Take 1/2 cup baking soda and sprinkle into drain
4) Pour 1 cup white vinegar into drain - this will cause the 4th grade science project volcano effect...FIZZ
5) Immediately cover drain with old towel/rag so all the fizzy goodness to clear drain stays in the drain. Let it begin to work its magic.
6) Boil a kettle full of water and after 5 mins of fizz action - remove towel and pour boiling water down the drain to flush the system through.
I'm not going to lie...the process is a bit messy BUT it really worked. No chemical stink and it's been running proper since then.