Wear them around the house. Also wear thick socks
Put on thick socks (thicker than the ones you'll usually use) and put the boots on. Blast them with a hairdryer for a while and let the cool down while still wearing them. I did this for a pair of ariats and it definitely made them more supple.
My custom Tuccis were fine immediately. Once you start riding you will be fine.
The best way to break in tall boots is to wear them as much as possible! Don't zip them fully for the first few days. If you want to break them in even quicker, you can go to a shoe repair store and have them stretched. Finally, rubbing alchohol can be used on the boots to make them stretch more! I've personally never used the rubbing alchohol, but my trainer has!
My custom tuccis were comfortable from the first ride. I rode in them the day I got them and they were surprisingly comfy. They were more uncomfortable walking around in them than riding in them! They broke in quickly too! Love them
Just wearing them will help. Even if you are just walking around the house, the heat from your legs will supple the leather. After you have had them on, do as Madison said and stretch your ankle down. After you have suppled them, them you should ride in them until they are fully broken down into what you would normally ride in.
I hope this helps
step on the edge of your mounting block and let your heels sink down, this will hopefully help :)