A milk bath obviously requires milk! It is good for the skin and creates that solid white perfection in the photos. But your bath doesn't have to be 100% milk. Even 1/4 milk parts will still give you that opaque liquid. I use full cream milk.
Add some texture to the milk, I like to use a combination of 2 colours of flowers and lemon slices. You could use any texture you like but you must think about how long it will float. To get the longest time for those pops of colour to be visible and not just sink to the bottom, choose flowers with a large surface area and cut the stems off as short as possible. For fruit, cut slices as thin as possible so they aren't too heavy.
Prepare the milk bath
Earlier in the day, let the milk sit on the bench until it is room temperature and then mix the milk in with WARM water. You are only going to get about 5 minutes of all the flowers (or whatever texture you are using) floating nicely on the top, so timing is really important. Choose a time of day when the child is not sleepy or hungry and allow them to have a toy and their favourite music playing. Test the water for temperature first and then sit or position the baby facing the camera, or at the angle you expect to photograph the at. You won't have time to change around position or to try and coax smiles out, so they need to already be having fun splash splashing in the warm milky bath. After the child is in the bath and happy add your texture ingredients and start photographing straight away to capture the joy!
Other tips:
Safety first, the best idea is to do this with another person to spot for slipping while you are busy taking photos, especially if the child is not sitting by themselves yet.
Be aware of the background. A natural setting or a studio look? Rustic or modern?
Make sure the light is even, most likely you won't have the luxury of photographing in golden hour so choose a shady spot.
Use interesting props. Find a cool looking vessel for the milk bath, find some material to place it on for extra depth, use left over flowers, a jug, fruit, anything that fits with the theme you have chosen to place around the base to set a more intricate scene.
Place a WHITE towel on the bottom of the bath for the baby to sit on. The bath might be slippery, scratchy or cold.
Get in the photo! Lots of mothers may not feel like being in the photo, you might be covered in splashed milk, but these could end up being your favourite, fun images together.
Let the little one just have fun. It doesn't matter if they don't look at the camera.