3 gallons of petrol
1x lighter or box of matches
1. Put on boots and lace them up
2. Pour petrol In to the bucket
3. Stand in the bucket
4. Strike match/ ignight the lighter
5. Drop still burning match in to bucket of petrol
6. Stand still
7. Burn to death
Or alternatively you could use tbe the search function you utter ****.
Hot water, wear them laced up tight (wel, tightish) and walk in them.
Also, use neetsfoot oil, including inside, and let it soak into the leather. Flex the leather lots with your hands too. Neetsfoot oil is a traditional thing, you can even look it up, but expect a hard job to get them shiney for a while.
Do it the Sven Hassel way with lots of urine, dead Soviets and SS Panzers roaring all over the place.
Or just wear them for longer and longer periods until they stop hurting.
Serious Note: Do modern boots need breaking in?? Even in the mid eighties if I got a pair of new boots I would lace them up and lead on, no problems. Mind you, I was a Cold War Warrior and therefore uber hard.