Seed bombs are balls of clay, compost/worm castings and seeds mixed together at a ratio of approx. 5:3:1. The clay can be dug up from the ground if you have access to that, or purchased from a craft store. We bought kiln dry terracotta clay from Riot Art & Craft, but we just let it air dry so that it wouldn't be too hard for the seeds to break out of! Seed bombs can be thrown into abandoned lots or garden beds, or just into your own garden. When the environmental conditions are right (i.e. it rains and the temperature is warm enough), the clay will soften and the seeds will sprout, with the organic matter giving them a good start to life. The clay and organic matter both also retain moisture, which helps to sustain the seedling in those first crucial days after germination if it doesn't rain again.
You can use all kinds of seeds, but we used a mixture of pumpkin, capsicum, sunflower, lavender, loquat, bean, bitter gourd and silverbeet. It will be survival of the fittest as to which seedlings survive, but this gives the bombs the best chance of success overall. If you throw them into your garden, you can always separate out the seedlings so that they all have plenty of space.
Now to spread the gardening love around by throwing these into spots that could use some greening!