Title says it all. I'm talking specifically about Central Yiddish אַ = /a/ vs. ײַ = /aː/. My native language is American English.
General advice; hopefully someone can answer with a response that takes into account the particular sounds and languages.
Find a pair of words where the mentioned sounds is the only difference, or a major difference, between them. Ideally find words that you can otherwise pronounce well and effortlessly. Listen until you can hear that there is a difference.
Practice pronouncing the words by speaking them out loud. Check your pronunciation against resources such as Forvo, if available. Practicing this develops the muscles you need to create the proper sounds.
Continue speaking in and listening to the language.