Simple answer, multitasking does not exist. What you believe as multitasking is different as it might look like. But let me explain it in a bit more detail.
Research and studies have shown that our brain is not able to handle several things at the same time. You can not work on two things parallel. But what about speaking to someone while you are driving your car. Or doing housework and listen to the radio. There are many more examples. This looks absolutely like multitasking. But what is happening in real.
A better word for it would be switch-tasking. Our Brain is not able to work on several things at the same time but it is able to switch between tasks really quick. You might realize this very simple if you try to read something and a fried is telling you a story while you are reading the text. Try to follow both. It is really difficult to understand both at the same time or even concentrate on both things . But what you can do is switching between one and another. This is a very slow example of what is happening in real. In other cases the same is happening but in a much faster way. Sometimes it will be that quick that we can not realize the difference.
How you can use this for work?
This is another example of focusing on the right stuff. Try to complete the first one and then finish the second. You should avoid the situation when you want to do several things at the same time. Also avoid interruptions. Stop the disruption until you have finished what you do at the moment. Turn of emails and put your phone on silent mode if you want to concentrate on a single task. The amount of brainpower is limited. If you switch to often you can not focus the power. It will be divided to the different tasks.
So keep in mind multitasking does not exist. Focus one thing after the other if you want to accomplish tasks quick and effective. You will see improvements and better results of your work really soon. Keep this in mind.