I fit boots for along time. Ill echo what people here have said.
boot should be too tight feeling out of the box. If they feel good, they will suck to ski in after a little bit.
pull liner, put foot it, no more than 2 finger (15mm) space between your heel and shell. this is a performance recreation fit.
REALLY fit boots with custom shell work end up less aroundd 8-10mm shell fit. I typically measure a 28.5 but run 27.5 shells and punch them out where needed. It takes alot of work, but I can hang out in my boots all day comfortably and the shell is so much more supportive, especially through heel an instep because it is that much closer in to your foot. If I tried to ski in a 28.5 boot these days I would feel like i was swimming. Conversely there are alot of 27.5 boots I literally can't make work for me; shells cant be stretched, shorter than average BSL, and so on.
While a bootfitter is incredibly helpful, if you dont have access to one you can stand on a branick board and get an idea of your foot width. To be honest, most of my fits were through visual analysis. Once you know what different boots feel like, you can see a foot, see a shell, and boots that are good fit options will have a shape to them that.
When you put new boot on, toes are gonna be crunched. That is easy to fix. A good custom footbed (The kind that gets built up and ground down by a fitter and costs well over $100) is one of the best investments in your skiing you can make, ESPECIALLY if you have arches that flatten out when you put weight into your foot. They help stablize your foot and pull it in slightly on all sides and up slightly in the heel. This also makes your foot a little smaller, so more room in boot. skiing improves greatly because the boottom of your boot you push into to turn is now 100% mated to the sole of your foot for instant power transfer. Toe boxes stretch out real easy too with the right tools, you can literally stretch the plastic of most boots to make more room. or they can grind away material. The biggest areas to pay attention to are the area around your instep (where your leg and foot met at the front of your ankle) Pinching or pain points in this area are the hardest to fix. Pain around the outer edge of the foot is typically a sign of a collapsing foot pushing out to the shell-custom footbed. Heels need to not lift at all when flexing boot.
Heat molding liners can help shorten the break in period, but it is NOT THE SAME as shell customization. A good boot fitter will make nesscesary adjustments (at least a few) as part of the purchase and you can return to them if problems arise. You can always toss new liners and toe and heel lugs on boots so spend the time and money now preseason to get dialed and youll be set for years.
Final pro tip for buying new boots: pre buy the thinnest ski (or just general athletic compression socks) you can find, and cut your toenails short. These boots will be the tightest they will ever be, and the volume of a sock or gnarly toe nails getting crushed into the front of the boot WILL throw off the fit.
