Heads come in many sizes and shapes. You should be prepared for the possibility that the helmet you are trying to fit may not be compatible with this particular head.
1. Adjust the fit pads or ring Most helmets come with extra foam fitting pads of different thicknesses to customize the fit. Fitting pads are too squishy to help manage energy in a crash. Their only function is to make the helmet fit better. For starters, you can usually remove the top pad entirely or use the thinnest ones. This lowers the helmet on the head, bringing its protection down further on the sides. It may reduce the flow of cooling air, but probably not enough to notice.
Adjust the side fit pads by using thicker pads if your head is narrow and there is a space, or add thicker pads in the back for shorter heads. You may also move pads around, particularly on the "corners" in the front and rear. Leaving some gaps will improve air flow. The pads should touch your head evenly all the way around, without making the fit too tight. The pads may compress slightly over time, but not much, so do not count on that to loosen the fit. The helmet should sit level on the head, with the front just above the eyebrows, or if the rider uses glasses, just above the frame of the glasses. If you walk into a wall, the helmet should hit before your nose does!
There are also helmets on the market that use a fitting ring rather than side pads for adjustment. With these one-size-fits-all models you begin by adjusting the size of the ring. Some of them may require the ring so tight for real stability on your head that they feel binding, but loosening the ring can produce a sloppy fit, indicating that the helmet is not for you.
With the helmet in position on your head, adjust the length of the rear (nape) straps, then the length of the front straps, to locate the Y fitting where the straps come together just under your ear. That may involve sliding the straps across the top of the helmet to get the length even on both sides. Then adjust the length of the chin strap so it is comfortably snug. If it cuts into the chin and is not comfortable, it is too tight. Now pay attention to the rear stabilizer if the helmet has one. It can keep the helmet from jiggling in normal use and make it feel more stable, but only a well-adjusted strap can keep it on in a crash.
When you think the straps are about right, shake your head around violently. Then put your palm under the front edge and push up and back. Can you move the helmet more than an inch or so from level, exposing your bare forehead? Then you need to tighten the strap in front of your ear, and perhaps loosen the rear nape strap behind your ear. Again, the two straps should meet just below your ear. Now reach back and grab the back edge. Pull up. Can you move the helmet more than an inch? If so, tighten the nape strap. When you are done, your helmet should be level, feel solid on your head and be comfortable. It should not bump on your glasses (if it does, tighten the nape strap). It should pass the eye-ear-mouth test. You should forget you are wearing it most of the time, just like a seat belt or a good pair of shoes. If it still does not fit that way, keep working with the straps and pads, or try another helmet.
Note: with a helmet that fits this well on a child, you must be sure the child removes the helmet before climbing trees and playing on playground equipment. Otherwise there is a risk of catching the helmet and being strangled! That doesn't happen in normal bike riding, even in crashes, but it can happen while climbing trees or monkey bars.