If you want a great summer display of trailiing Petunias, then you really do need to get your plants ready now. We normally buy plug plants in March/April and grow them on in pots ready for planting out.
As the plants grow on, it's very easy to take cuttings ( note some plants are protected apparently by the breeders, and you're not supposed to propagate yourself !, certainly not for selling on). We normally just snip off 2" of healthy growth, just below a leaf joint. Take off the lower leaves and put into a small pot of compost ( well watered previously). Make sure the soil is firm up against the cutting. Keep them on a light windowsill until they get going ( normally around 3 weeks). You can put in a propogator if you want, but they seem to do just as well in the open air.
We tend to go for the Surfinia Petunias, which are from the Hedgiflora( spreading) category. You only need 1 plant in a small hanging basket, as they really do spread out. Just keep them well fed, and deadheaded then you will have a great display through until Autumn.
Our favourite last year was "hot pink", it seems to stand up well to all the elements can throw at it. We even overwintered a couple of plants in frames outside, and they've survived.