For Cities basically the same principles apply as for organizations. First and foremost you would want to ensure the quality of your offsets, that they are truly additional, not double-counted and promote sustainable development in the area where they are implemented.
Depending on the amount of offsets you are planning to purchase in order to eliminate the remainder of your emissions, you might consider going through an offset provider which will be quite convenient. Choosing a particular project that is easily understandable and with obvious benefits to the climate, local people and the environment will help to include your citizens and communicate what offsets are and how they work.
Larger cities might be interested in the option of purchasing directly from project developers or even create their own projects as described for large organisations.
Through the Kyoto Protocol mechanisms countries are bound to the compliance market if they want offsets to be accounted on their emissions balance under the legal framework.
Countries play an important role in bringing forward the whole system of carbon offsetting. In order to improve the whole system for mandatory and voluntary offsets a country can do even more than a city to raise standards in the industry, both by what it decides to do nationally and by working for effective international regulation. Obvious examples are the countries which were the first to join UNEP’s Climate Neutral Network: Costa Rica. Iceland, New Zealand and Norway.
The Holy See (Vatican) is one example of willingness to explore the potential of offsetting, planting a forest in Hungary to compensate for carbon emissions from papal flights. Any city or country can also exert some regulatory power over offset schemes, for example by requiring suppliers to choose only those proven to work.