The most practical way is to start by studying the meanings of each cowrie shell and the basic positions: open, closed, on its side. each of those states has a different interpretation, so you need to learn these symbols first before trying to read a whole cast. grab your grandmother's set, separate each shell, study which is which, and write down the meanings. there's tons of content online about this, from videos to Facebook groups where experienced people share their knowledge.
once you've got the basics down, the next step is to ask clear questions before you cast. it's not just about grabbing and throwing the shells: you need to formulate a specific question, focus on it while you cast, and then organize how the shells fell on the cloth. some fall open, others closed, some turned over, and all of that changes the reading. the arrangement matters too - some end up further forward, others toward the back, and all of that goes into the interpretation.
start practicing by asking yourself simple questions: "will i be able to do this?" or "what's my biggest challenge right now?". cast and write down the results to see if the answers make sense later. as you get more confident, you can read for other people. remember that interpretation is pretty intuitive in the end, so don't get too hung up on the exact meanings - there's room for your intuition to play a part too.