Based on the definitions of several dictionaries, here is an adapted definition for "pronunciation" as follows: 1) the act or manner of pronouncing (manual or vocal) words; utterance of speech-language or sign-language, 2) the way a word or a language is usually articulated (signed or spoken). Though, the term "sign production" is generally used, which is equivalent to "pronunciation".
It might be easy for English-speaking students to learn and pronounce Spanish or French but difficult for them to pronounce Navajo (one of Native American languages) or Xhosa (one of African languages). Students are accustomed to pronouncing certain ways that are similar to their native language. But, they may find it more challenging when they are unfamilair with remote pronunciation of another language.
As for learning Ameslan or ASL, students learn not only another language but also another language to pronounce in another modality (from vocal-auditory to visual-manual). Students learn to develop motor control of their different parts of the body (fingers, hands, arms, mouth, eyes, eyebrows, and head) that they are unaccustomed to. They are accustomed to speech produced by a coordination of the tongue, lips, glottis, vocal cords in larynx or voice box, trachea or windpipe, nose cavity, and lungs. In addition to pronunciation, students also learn to perceive subtle differences in a person's ASL signing through eyes instead of ears.
In addition to learning a language from a book or a website, socializing and studying in class are two other ways to learn to pronounce ASL correctly. Teachers, colleagues and friends all who are ASL natives can usually help non-native signers pronounce correctly whenever they see.** Learners should be aware that many ASL words are incorrectly pronounced (or taught incorrectly) by non-native (ASL as a second language) signers.