How to make comfrey feed:
Use a water-tight container, preferably one with a tap at the bottom, although you can scoop the feed from the top. Stuff with comfrey leaves and add water to cover the leaves. Add a lid and leave. The fermentation process can take from a matter of days (in summer or in a greenhouse) to months (outside in winter). The longer you leave the feed the more concentrated it will become. You can keep a container of feed on the go by adding leaves and topping up as you use it. Alternatively do not top up with water, but just apply a weighted board to the top of the leaves.
If you have patience, and a container with a tap, you will get nice strong, evil smelling comfrey feed that will continue to build up as long as the leaves are still rotting away. Use the feed either straight or diluted on tomato plants or just about anything else in the garden. It could be used as a foliar feed, but you might get your sprayer blocked up. Comfrey is a dynamic accumulator of many of the essential elements for plant nutrition. Most significantly it is high in potash, which makes it ideal for feeding tomato plants, especially as they begin to set fruit. I have used this malodorous feed for several years, and particularly to grow organic tomatoes. I no longer water my tomatoes at all. All they get is diluted comfrey feed. That's a job that you know have been doing by the smell that follows you around for the rest of the day. But the taste and vigour of the tomatoes is a just reward.
Finally it is worth bearing in mind that the use of a green manure plant like comfrey is not a free lunch. Even comfrey beds can become exhausted. One farmer I know of keeps sheep up the hill from his comfrey bed, and the run off from the field they manure keeps the comfrey bed healthy. He also makes a good business out of selling the bottled feed, and root cuttings of the comfrey to gardeners. You could try, once every few years, manuring your comfrey bed in the winter, or just using the bed sparingly and allowing a high proportion of the leaves to die back in situ.