New leather work boots take 40 to 80 hours of wear to fully break in. That's two weeks of full work days, give or take. There are tricks to compress that timeline. There's no trick to skip it.
First — wear them indoors before you wear them outside. Two evenings, three hours each, around the house with normal socks. The leather warms to your foot temperature and starts to mold. Skipping this step means your first day on a jobsite is your first day breaking them in, which is when blisters happen.
Second — get the lacing right. New boots come laced loose. Re-lace them so the tension is even from the lowest eyelet to the top — not loose at the bottom and tight at the top. Even tension means even pressure on the foot, which means the leather molds to your foot's actual shape, not to whatever shape the lacing is forcing.
Third — wear thicker socks for the first few days, then switch to your normal socks. The thicker sock fills the boot fuller, accelerating the molding, and it cushions the spots where the new leather is going to push hard. After three or four days, switch to your work sock — by then the leather has given enough that the thinner sock won't slip.
Fourth — use boot stretch spray on the hot spots, not all over. Identify where the boot is rubbing — for most people it's the back of the heel, the side of the little toe, or the top of the instep — and spray those specific areas, then walk in the boots until they're dry. Targeted softening works. Soaking the whole boot in water, the trick from the army boot days, is overkill for modern leather and can damage the lining.
Fifth — if the heel is slipping at the back, that's not a break-in problem, that's a fit problem. Slipping means the boot is too big in the heel. Walking through the slip will give you blisters and won't fix the fit. Bring it back, exchange it. We'll do that.
Sixth — accept that some discomfort is the boot doing its job. Leather work boots are stiffer than your old boots because the structure is what holds your foot in place under load. The stiffness softens. The structure stays. That's the trade.
If after two weeks of regular wear the boot still hurts the same way it did on day one, the boot doesn't fit you. That's different from breaking in. Bring it in, we'll figure out which it is.