Articles such as "How to Resole Shoes", describe the process for dress shoes. But something more radical is needed for repairing school or walking shoes with deep treads.
The Replacement Heels you can buy for shoes are usually a thin sheet of plastic or rubber, with little tread. This is not enough for a heavy duty school or walking shoe. The shoes can have so much wear the replacement is not thick enough.
Shoes with thick soles can be repaired with a more radical approach. Shoes normally wear out at the heels first, so you can use the sole from an old shoe to make the heel for another.
First use a small wood saw to cut the sole from an old shoe. You may have to saw through the leather of the shoe to get at the sole and then use the saw to cut the inner lining from the sole of the shoe. Leave the sole larger than the heel you are repairing, as you can trim it to size later.
Now use a saw to cut the bottom off the heel of the shoe you are repairing, to match the thickness of the sole you are using to replace it. You may have to cut 5 mm, or more, from the heel. Don't worry if the heel has voids in it, or is not completely flat.
Normally Contact Adhesive is used for gluing shoe soles, but it requires very flat surfaces to stick. Something thicker is needed for more radical repairs, where the surfaces are uneven and there are holes to fill. I found that the Silicone Sealant, as used for windows and around bathtubs, works well. This sticks to rubber, plastic and leather and also is thick enough to fill in holes and uneven surfaces.
Select a colour of silicone to match the shoe sole (usually black). Buy the large corking gun tubes of sealant, as you get a lot more silicone for your money this way.
Make sure you are wearing old clothes and have gloves on, as the silicone sealant is very messy, particularly in black (it sticks well to clothing).
Smear a generous amount of the silicone to the shoe heel, filling in any voids (some shoe heels are hollow). Now smear silicon on the replacement heel and press it in place, lining up the tread.
Leave the sealant to dry, which may take up to a week. It is important to leave the sealant to set all the way through. It may look set around the edges you can see but still liquid in the centre. If you try to walk on the heel in this state it will slide off the shoe, which could be dangerous.
When the sealant is set, carefully saw the excess from around the heel. You can clean this up with steel file.
The result will not be as pretty as a new shoe but will be very serviceable.