This PDF contains an 11-page fully detailed instruction manual of how to smock, originally from a post-WW2 publication.
Details of how to gather up fabric from the reverse side is given, and then the process of gathering up the fabric using long threads is explained. The actual smocking stitch is worked from the right side and the gathers are not released until the embroidery is finished and the garment completed and ironed. You need to begin with a width of fabric around 3 x the finished required width. I recommend you mark the approximate edges of the pattern on the back before working the smocking, and cut it out somewhat wider, then finally cut your bodice to size after the smocking is completed, just in case it is narrower than you planned. To get the smocking to the right width you need to gather up the smocking dots to rather narrower than the required finished measurement!
I learned how to do this at school in the Fifties and it is not difficult, especially if a 12-year old could do it. The trick is to keep the embroidery threads very relaxed and never to pull them tight. Working on a squared, gingham or dotty fabric helps to keep the rows straight on a first effort (and then you might not even need to mark dots on the reverse). It is a lovely embroidery to work, quickly finished. Use a bold thread, stranded embroidery floss, or a soft 'crochet-type' cotton - white on a coloured fabric is very 'classical'.
The finished effect is magical, it looks very expensive and adds a lovely touch of tradition to your baby and toddler clothes. In the 70s we had in the UK a revival of dresses designed like smocks and of course real smocking was essential - machine imitations were not a patch on the real thing. Smocking has to be worked by hand, it cannot be copied by a machine.
I have left in the detail of the dressmaking patterns recommmended, as they give the yardages of fabric. 1 yard is 36 inches or 0.9 m. Dressmaking fabrics in the UK were woven to be 1 yard wide in those days.
To save you time having to manually measure your rows of dots I have included a PDF of a grid suitable for toddler/baby clothes. To transfer the dots to the back of the fabric, try lightly rubbing a soft pencil over the reverse side of the paper, laying the paper wrong-side-down on the reverse side of your fabric then pressing the dots with a blunt pencil. The dots need to be invisible on the right side as the gathering is done on the wrong side and the embroidery smocked on the right side. It doesn't really matter if the dots never wash out as they are invisible from the right side - just don't make them too bold, they only need to be visible for the initial 'picking up' and are never used again as the embroidery is carried out from the right side.
This tutorial was found in my extensive library of needlework, knitting and crochet books and magazines, dating back to the late 19th century, many inherited from my mother, grandmother and mother-in-law.