Well, it's not exactly liquid nitrogen.....more commonly referred to as compost tea or manure or compost extract and it can be created from any composted manure or directly from rabbit or llama manure. Other manures should be composted or aged well before using to destroy pathogens and reduce the potential for nitrogen burn. As with any composted substance, it will contain a range of plant nutrients - rabbit and llama manures tend to have higher nitrogen levels than most with less ammonium.

The proportions are not terrifically important nor is the length of time it steeps. What you are looking for a is dark colored liquid - essentally a water soluble fertilizer. Avoid steeping so long that an unpleasant aroma develops. This is indicative of an anaerobic situation, which can encourage the development of harmful bacterial properties.

Here is a link that might be useful: compost or manure extracts