I am surprised to see that no-one has yet mentioned understanding and employing perspective.
Using the 2D grid recommended in other answers will enable you to achieve precise interrelation and proportions between the details of the subject. But it will not teach you about why those details are where you see and depict them. Thinking in 3D, according to the framework of linear perspective, does that.
For me, anything in life-drawing (and especially drawing from imagination) is getting conscious about the structure of the subject in 3D, and then demonstrating that structure on the paper as best as I can (translating it back into 2D). First comes demonstrating just the 3D structure, then comes building up the details of the subject on top of that preliminary sketch of the basic structure.
In the beginning, one does not need to make it overly complex and demanding, like a human figure & head.
Most often, people start learning and practicing perspective by drawing cubes and similar abstract objects. Even if it feels boring and uninviting, this part is hard to skip.
But one can quickly transition to some real-life subjects, which could still count as simple. Drawing a loaf of bread with a kettle and a teacup next to it could already be a helpful subject. Or, put half a pair of shoes next to a vacuum cleaner. Anything that's made up from few basic forms, but already has a lot of curves.
Look for, and reproduce just the basic shapes that they consist of. When you feel that you got their spatial arrangement and proportions right, jump into developing some detail on them. Create different compositions out of them, place yourself sometimes nearer, sometimes a bit further away from the subject.
After you have gained confidence disassembling these random objects in your head to their 3D essentials, then re-assemble them on the paper, you will have the basis to handle life-drawing, including the human head, with a similar thinking.
A few lessons from Stan Prokopenko to get you started:
After these, you could look up something on YouTube on the specifics of 2-point and 3-point perspective.
For life drawing specifically, learn to measure proportions with your pencil.
After you have developed a sense for thinking and sketching in 3D, and you can fit things successfully into a linear perspective framework, you can still bring forth the grid that others recommended: it may help you to achieve higher precision in quicker time. But experience in working in 3D will deliver an essential contribution towards more convincing results.