Single to double chainset conversion Can I change a single chain ring to a double?

I have an old steel frame with a narrow bottom bracket shell 66mm and a single 50 tooth chain ring. I would like to add a 36/40 inner and want to know what is needed and how to go about this.

Many thanks
You need a bunch of things. 


left hand shifter, or a left hand replacement brifter if your bike has integrated shifters
front derailleur mechanism with a band-on (aka bolt-on) clamp mechanism and diameter of the same size as your seat tube
new inner gear cable and cable housing for the front mech
a pair of cranks that have the same length, and same size pedal mount hole as your existing pedals
the right hand crank should have chainrings sized to suit your need.  Since it takes a 50T now, smaller toothcounts should fit fine. Cranks should have the same BB interface as your current BB axle (probably square taper but others are possible)


Depending on the details you may also need


new bottom bracket axle with more length
bolt-on frame cable stop for the seatpost.  May need to be angled, OR
an "under-BB" cable guide.  Generally it is a piece of plastic, and is screwed into a hole in your BB
bolt on downtube cablestops or holders (if cable zip-ties aren't to your taste)
new grips/bartape.


Sourcing parts

If you have to buy all this new, it may be cheaper to buy another bike.

If you have access to a donor bike, something that has been crash damaged could be good.

Or start exploring your local bike cooperative or ebay/CL/gumtree etc.