What can I do to encourage the worms to return?
What can I do to encourage them to grow?


Add much organic material to the soil, preferably fairly finely divided.
Some watering will help if you are in drought. They don't want it to be wet
just with some soil moisture, they will not thrive in really dry or really
wet soil.

Do I need to 'worm farm' and then introduce them to the garden?


Not necesarily. If you have any worms they will spread and multiply by
themselves but you can help them by taking some of the good soil with worms
to areas that you have just enriched. Also many worm farms are compost
worms not earthworms, the two are different and compost worms will not
usually survive in the soil for the long term.

David