Your first step is to locate the spots where there is substantially more than
average pressure on each foot. For example, these might include the inside and outside
ankle bones (pictured), the big and small toe, the arches (sides of the foot),
and/or the top of the midfoot. You need to make space at these points first so you can
judge the rest of the fit. It is a good idea to mark them with a small piece of
athletic tape while your foot is still in the boot.
Do not mistake excess pressure points with points that hurt because of a blister. Blisters are sores caused by friction from rubbing and the heat it generates, and occur at points where you have too little pressure.
Stretch each of these points with a "ball and ring pliers" (also called "hoke and ball pliers", or "bunion stretcher"). This is a locking pliers that has a ball at one side, and larger ring on the other. The ball goes inside the boot where you want to push the leather out. The ring goes outside, over the place you marked with tape. To avoid marring the boot, place a piece of cloth under each side of the pliers. Squeeze the pliers, while tightening the set screw to lock it. (Most are not built strong enough for the set screw to be used to tighten the squeeze. But you could use a clamp or vice to apply more pressure, if your hands aren't strong enough, but don't use so much pressure that the leather is damaged. Do not do as I once did, and apply your body weight to your hand to do it - something in my wrist was strained or sprained for a few days.) After an hour, the leather has stretched a little - readjust. And again, after an overnight stretch. After a day or more, you will have a fairly complete stretch.
It helps to heat the leather at the spot a little (maybe 140 degrees F) first, but not enough to burn the leather or melt the glue. In theory you can go to heat molding temperatures, but that would alter the shape of the rest of the boot.
Neverdull44 on SkatingForums.com used an innovative solution that didn't need fancy tools: "I heated up with an air dryer for about 5 minutes, moving it back & forth so as not to burn it. Then, I put the screwdriver head [My note: she means the blunt handle] where I wanted a punch and held it for about another 5 minutes. Then, I put the boot on & lace it all up & stand in it." She credits the idea to Tim Burt, an expert skate technician at the "incredible SPORTS Pro Shop". She was working on Edea boots made of composite materials (Warning: as mentioned here, Edea boots are easy to destroy if you heat mold them wrong.) - probably carbon fiber plus plastic resin, like high end hockey boots, not leather like most figure skating boots, but the same ideas should apply to leather, though it might take a little longer. I haven't tried this, but if you need to hold the stretch longer, you might improvise by putting a hard rubber ball inside the area to be stretched, a thick cloth on the outside, and use a wood clamp to push them together. (Don't apply enough pressure to tear the leather.)
It also helps to first work in an oil to the inside, like Lexol Boot Conditioner. But oil should not be applied to suede - it will discolor and destroy the knap. Water (with a little alcohol to dissolve the oil in the leather) stretches leather even better, but water cracks and stiffens leather, so let the boot dry after stretching, and work oil back in. Skate boots are so thick you will never get oil back in to all the layers of leather, so many experts prefer to avoid moisture. Do not soak boots in water. The boots were probably made by sewing and gluing the leather over a last that matches the assumed foot shape (but perhaps with extra space to allow for pointing and flexing the foot??), using pressure, heat and moisture, but boot makers know what they are doing.
Some people say boot size can be expanded a bit by using them with wet socks.
It is hard to completely eliminate the pain from high level boots, because they are so stiff. Many people buy boots that are too stiff for them, because they think more expensive boots must be better. Really stiff boots are only needed for high level jumps, though some stiffness is needed to support the very deep edges ice dancers favor. You can often order ice dance boots with softer fronts, tongues, and lower backs, but with somewhat stiff sides, which is a good compromise. You can also cut V-shaped notches between two levels of lace hooks, about midway up the foot where the ankle bends, to make bending easier, but that may help cause premature break-down.