The terms, foremilk and hindmilk are sort of misnomers and can lead to a lot of confusion. Your milk is made with a consistent amount of fat, but as it sits in the breast, this fat settles out of the milk and clings to the ducts. Your milk is not homogenized, so just like raw milk sitting in the fridge, the fat does not stay mixed in the milk.
When a baby nurses, the let down forces some of the fat back into the milk. During each let down, there is progressively less milk and more fat concentration, until baby is receiving milk that is being made in the breast as he/she nurses and this milk is going to have the amount of fat that it was created with.
Feeding the first side until the breast softens, allows baby to get as much fat as needed. For babies who have difficulty nursing and need extra help, massage and compressions will increase their ability to get more let downs and more fat.
The amount of fat a baby will get in a feeding will naturally vary across the day, depending on how full the breast was before feeding and how long it has been since the last feeding, which determines how long the fat has had to settle out of the milk. As long as you let baby nurse until satisfied before switching breasts and as long as you do not have a significant oversupply, you do not need to control this process. Baby is pretty good at figuring all this out and will ask to switch sides if they want a different kind of milk.
Laura Spitzfaden, LLLL, IBCLC