Effective microorganisms mother culture can be used straight from the bottle, or it can be activated first.
Note that EM is sold under many brand names (you may have searched for em1, em-1, emro, biosa, efficient microbes, etc.), some better than others.
This one from SCD called 'ProBio Balance Plus' is on par with the best of them, made by people who have studied under Dr. Teruo Higa.
How much do you need? Application rates and dilutions are the same for both EM mother culture and Activated EM. They can vary quite a lot, so refer to the product page for details.
Generally, one liter of EM, diluted with 100 liters of water, covers 10,000 square feet. One gallon covers an acre. Or, two teaspoons (10mL) of EM diluted in one liter of water, is good for 100 sq ft. That said, most people buy mother culture to make their own activated product, which means they end up with 20 times the volume and therefore, 20 times the coverage. This makes it a very economical choice, worth the time and effort of activating.
How long does it keep? Mother culture is good for 18 months to two years if stored correctly. Keep the cap on tight, and store at cool room temperature, protected from frost, heat, and direct light. Mix and use only what you need to apply the same day. Diluted mixed product does not keep long.