VOICELESS GLOTTAL STOP [ʔ]
To pronounce alef, constrict/stop the flow of air low in your throat. For those familiar with Arabic, it is pronounced like the letter hamza (ء).
GEMINATED VOICELESS GLOTTAL STOP [ʔʔ] or [ʔː]
When alef has dagesh in the Bible, it should be pronounced with gemination (i.e., held for a longer duration).
SOURCES AND FURTHER READING
Khan, Geoffrey. 2020. The Tiberian Pronunciation Tradition of Biblical Hebrew. Cambridge: Cambridge and Open Book Publishers. §I.1.1.