Not an easy choice
Ski-boots are possibly the single most important part of a skiers outfit - they directly influence the comfort, safety and level of control of the entire experience. The market is full of recognisable equipment manufacturers, there are also several brands specializing in ski-boots exclusively. The variety of choices and designs can frequently become overwhelming. How, then, does one choose the correct pair of boots for himself?
Outside the obvious question of size, the first aspect to consider when choosing ski-boots is the rigidity index (called flex). The higher it is, the more rigid the boots. The more rigid the boots, the higher the control and precision whilst skiing. Less rigid boots are more forgiving and definitely more comfortable. Beginners should seek softer boots initially (men: flex 60-80; women: flex 50-60) - they're easier to get acquainted with and the skiing itself takes less effort. Slightly more experienced skiers often choose medium-rigidity boots (men 85-100, women 65-80), the advanced ones prefer rigid (men 110-120, women 85-100), and experts and professionals go for extremely rigid boots (men 130+, women 110+). It's worth noting that the rigidity index itself is not standardised and every manufacturer uses his own scale. This makes it crucial to try the boots on before purchase, as flex 100 boots of one brand can turn out to be more rigid than a flex 110 pair from a different one.
Some manufacturers also provide information about shape (narrow, medium, wide) and the lining. These are also subjective and thus not always useful. Experience shows it is typically worth it to try on different pairs and to seek advice from the qualified store personnel. To increase comfort, some manufacturers offer boots customised for the individual users feet. However, these thermally molded models are typically more expensive.
Take your time when choosing the correct pair of ski-boots - a rushed choice is rarely a good one. The process may seem dull and can be surprisingly slow (up to 2-3 h), but it is the only way to end up with equipment that is both safe and comfortable.
Once settled on the correct rigidity, try out several different pairs from the store. This will help visualise the lack of uniformity amongst different brands: the variance amongst similarily-marked pairs in shapes and sizes. Even the same models from different years can turn out to be shaped differently despite being labeled as same size. In fact, it is entirely possible that the entire store won't have an optimally-sized pair of boots for us - which highlights the futility of choosing ski-boots online, even if we know the "correct" size.
Trying them on
Make sure to wear your skiing socks while trying the boots on - using regular socks will lead to a poor choice. Make sure the socks are smooth and straight. Once the boot's on, fasten the buckles. If you can't fasten the buckle on the last indent it means the boot is too wide. Optimally you should have a comfortable and painless fit on the penultimate indent. No inserts should be necessary - if they are, it means we've chosen too big a boot.
Most stores offer a testing station, where boots can be set into floor-mounted bounds. There we can check how the feet behave in a skiing position, as opposed to standing normally. If no pain is present during testing, check for loose fits. A well-matched boot will have your big toes slightly pressing against the front whilst standing upright. When in the bounds and leaning forward, the feet should move back a little and the toes should lose contact with the front of the boots. Pay attention to the heels - they shouldn't be able to move more than a few mm vertically. Also important is the boot's width. The foot mustn't feel loose on the sides, yet it cannot be held crushingly tight, either. The shanks are much the same, however, while some boots can have their width adjusted, few pairs offer such customisation for the shank, so make sure the boot's height is the right one for your foot. Bear in mind that any discomforts whilst wearing ski-boots can take some time to manifest, so spend several minutes walking around to check for possible pain and numbness.
And that's it - in thoery. In practice, the process is time-consuming and depends heavily on the person in question. Definitely spend some time actually wearing the boots - merely being able to put them on and off again painlessly doesn't mean we've chosen the correct pair. Be patient and dilligent - even the tiniest discomfort will strike back with vengeance on the slope, where the temperatures are lower, the boots less elastic and blood circulation in feet poorer.
Bought! Now what?
Once the purchase is done we should spend some time (say, an hour) wearing our new boots for the next few days. Should discomforts arise during that time, they should be returned and a new pair chosen. This makes it worth it to make a note of models we liked during the initial testing - if anything turns out wrong, we can then come back for another pair from the list. Most stores allow returns for a period of time after purchase, provided we haven't removed any of the tags and the boots are unused - usage being actual skiing, not merely walking around the house, thankfully.
If we elected to choose a thermally molded pair for ourselves, it might be worth it to first take them for a short trip so they can get an initial fit for our feet. Only then should we take them back to the store for the final mold.