Coat the seeds with dry compost, then clay. Add water, a little bit at a time, until the mass is easily 'kneadable'. Form small balls (~1 cm) and leave them to try for about 2 days. Spread in the city by throwing onto unused lots, parks, next to canals or rivers.
This depends for what purpose you're using the seedbals. For a small ecosystem in one ball, choose a mixture of 2-3 grasses, 2-3 flowering plants, 2-3 shrubs, 1-2 ground covers (clover is an excellent nitrogen fixer), 1-2 bushes, 1-2 small trees, that can form a plant guild. Preferably look for seeds that are endangered, edible by humans/animals, loved by bees and butterflies, good for keeping the water levels appropriate for other plants, are able to control pests, purify air and are not overly invasive. If you are planning to throw balls onto roofs, or close to buildings avoid trees and bushes, as well as plants with deep roots. Best is to choose plants that can help each other grow, such as companion plants or plant_guilds.