Repetition is key! Use the signs you’ve learned as often as you say the word.
In my experience, the best way to teach babies sign is the “I do, We do, You do” method of teaching. I do this by signing the word and pointing to what I’m talking about. Then, I sign it again, saying what it is, with their hands, on their body. I repeat these everytime I use a word that we are learning the sign for. Soon, they will be able to sign it themselves.