to give a little critique to the process.
Practice getting your feet in the liner and into the boot and the liner settled and boot buckled before you throw the liners in the oven.
practice this alot, then when you have it down pat, time youreself. Then when you actually put the liners in the oven, space the time you put them in by whatever this amount of time was. That way you won't overcook the second liner.
Have a second person there to open up the shell for when you slide your foot (in liner) into the boot.
When you slide your foot into the boot slide your toes to the front and shove your heel back. Buckle the boot tight, tighter then you think, then tighter, especially over the instep. Then stomp down hard and flex the boot forward driving your heel back.
When standing in the boots as the cool, stand on an incline with your toes higher then your heels, you can use a piece of wood for this. Stay flexed forward.