Lather boots in baby oil (provided you would be okay with the current color becoming darker) and compress the boots with the force of your hand or foot.
In order to stretch leather you have to put some oil on it. This is sold in stores for sure for baseball gloves. You should then put the boots on and rub the oil on the outside to stretch them out or you can put something in them around the same size.
Oil for baseball gloves is good for many leathers. However, there is a high probability that it will permanently darken the boots. Be careful. There are specific oils for softening leather boots/shoes that may be more color safe. Check out your local shoe repair store. posted by Anonymous
The best thing to do is to soak the leather boots in oil and that way it will soften the leather to be able to stretch them. You can take popcicle sticks and put them in the boots until the boot is stretched to where you want it and that way you know it is where you need them.
You can stretch out leather cowboy boots by purchasing plastic foot molds that can fit inside the boots. Leave the foot molds in the boots when you are not wearing them. Another way is to just frequently wear the boots.