You have to do two main things to smock - gather the fabric, and then add topsitches to hold the pleats in place.
I tried out 'smocking dots' - iron-on transfers in a grid pattern that guide the gathering stitches, as I am almost incapable of accurate measuring!
Smocking dots transferred onto fabric
Once the measurements/dots have been made, you sew a number of parallel threads for gathering the fabric.
When you've added the required amount, you pull them to gather the fabric into pleats.
Once the pleats are in place and the guide threads tied off together to hold them there, you can add the actual smocking stitches. So far I've tried outline, cable, feather, diamond and honeycomb stitches. Once the stitches are finished, you can remove the guide threads. The pleats are now held in place by the smocking stitches, and have an elastic quality.
From top left, clockwise: feather, outline,cable,diamond and honeycomb stitches