Just cut an existing hedge. Any cutting with a bit of stem and leaf will do. Just bung it in the ground.
Many will die, thats why you plant them close together, but most will grow.
You could buy some growth promoter, its in a white powder and dip the cut into it,
You could pot them, the compost will provide better room for the roots to grow.
These things will undoubtedly give a better % of successful plants. But they are both work and cost. Kind of defeats the purpose of planting cuttings.
Now, or over the next month or two, is the time for it. Worst of the weather over and all the growing season ahead to get them started. I have done this myself in the past. It certainly takes a bit longer for the hedge to set, in 12 months time you will think they are not thriving, but by Sept 12 months you should be looking good.
I will try to post a photo later showing where this has been done near me recently.