Give in whatever way suits you! Some B1G1 businesses link their products and services to specific projects and give a fixed amount per transaction. Others allocate a (small) percentage of revenue to giving activities each week/month/quarter and have their teams choose the projects. Every business is different, so choose an amount your business can easily afford but is enough to make the impact you want to see.
Here are some examples of how your business can give through B1G1.