I took a college class but I know some people who are largely self taught and learned mostly by networking with other bryologists. A good book is very helpful but the best one to use may depend on your location. Howard Crum's Mosses of the Great Lakes Forest is the one I use most of the time.

The Bryophyte Flora of North America website hosted by the Missouri Botanical Garden can be useful but it is incomplete.

The important things to know are the terms used to describe moss features so you can use keys. Things like acrocarp/pleurocarp, costa, alar cells, whether cells are papillose or not. A microscope is also indispensable for identifying mosses.

If you have not found this one already, I find the following link to be a good introduction to moss identification.

Here is a link that might be useful: http://ohiomosslichen.org/

Thank you. I have actually seen all of those resources before, but perhaps I haven't looked through them thoroughly enough. I suppose I'm used to vascular plants which can be easily identified with readily available dichotomous keys and species lists.