The way I learned was to be around deaf constantly.
Sign Language is not something that can be learned A to Z like english or a collection. Its actually a living language that has differences depending on where you are in the USA and sometimes the world. If you are with a deaf and the two of you are communicating you are learning.
Its a journey. Starts rather slow but at some point in time you will have quite a bit. It includes everything in it's structure. Not just about a subject but also time, gender and who and where and what etc. IF you can translate say the Declaration of Independance into ASL you can pretty much toss much of the writing on that paper. But keep the meaning and be understood.