Been there, done that, got the blisters to prove it. What saved me was wearing my boots while doing chores at home, like vacuuming or just walking around. I also used a hair dryer on warm setting to soften the leather in tight spots, just be careful not to overheat them. After a few days of that, they molded to my feet pretty well. Don't rush it too much though, or you'll end up with more blisters.
That idea of wearing boots at home with thick socks can make things worse from what I've seen. It stretches the leather in weird ways if you're not walking like you do outside. Even @susanb19's hair dryer tip might help fast, but it can hurt the boots if you're not careful. I got better results by wearing them on quick errands first, so they shape to my actual steps. Using thick socks inside tricks you into thinking they fit, then you end up with more blisters in regular socks. Really, breaking them in slowly with normal use works better than rushing it at home.