I always just think of babies as being little people. Just have a chat with them like you might with anyone else except the swearing (and accept that their responses might be slightly limited)
Oh and never the stupid high pitched baby speak voice with babies and toddlers, babies like a nice deep voice, and even quite young children hate being patronised by people talking to them like babies.
Although with our baby, ‘talking’ to her is mainly really about providing yourself as a handy climbing frame, just now as I was writing this she climbed up me and over my head.