This is a commonly asked question and the answer is simple...all of the time! Start from when baby is in the womb and take every opportunity to talk to your baby.
A few things you can try:
Pause and listen to their responses as this will help them to learn about conversations. Make sure you mimic the structure of a conversation as this will help them to learn how a conversation happens.
Talk about how your baby is feeling. Read their facial expressions and body language and discuss how they are feeling based on this. For example, "you're excited because we're going to the park" or "perhaps you're upset because you're tired.
It's ok to talk about positive and negative emotions as this helps them to talk about and share their feelings in later life.
Babble back to your baby any time they babble to you.
Narrate what you are doing. Walking down the stairs, the steps you take to change their nappy etc. The more you talk to them, the more they will learn.
Narrate the things you can see whilst out walking, in the supermarket or even sat at home.
Repeat words back to your baby. If they are saying the incorrect word, just say the correct word several times over. Don't say no or tell them they are wrong as you want to keep encouraging their attempts at speech.
Aim to read to your baby every day. It's never too early to start and it's a lovely thing to do when baby is in the womb too. It's a particularly nice way for your partner to bond with baby before they are born. A blog post about reading is coming soon!